The family Larnacida immediately follows the Larcarida as the next simple group of all Larcoidea; some genera of both groups (such as Larnacalpis and Coccolarcus, or Larnacantha and Larcidium) may easily be confounded from their being so much alike. In both the lentelliptical shell is composed of two concentric shells, an inner (medullary) and an outer (cortical) shell. But the connection between these shells and the construction of the inner shell is quite different in the two groups. Whilst in the Larcarida the medullary shell is connected with the cortical shell simply by radial beams, here in the Larnacida this connection is effected by two latticed lamellæ, which are the lateral wings of a transverse girdle. Therefore we encounter here for the first time that peculiar mode of growth which characterises the greater part of the Larcoidea, but particularly the Pylonida. But whilst in the Pylonida between the three crossed lattice-girdles, remain four open gates, here in the Larnacida the gates become closed by lattice-work; the lentelliptical cortical shell becomes perfect.
The most simple genus of Larnacida, and no doubt the common ancestral form of this whole family, is Larnacilla (Pl. [50], figs. 1, 1a, 1b). The most important shell of this typical genus is composed of a simple lentelliptical medullary shell and of three elliptical latticed girdles surrounding it, perpendicular one to another. These three "dimensive girdles" lie in the perimeter of the three dimensive planes, the minor (and first) in the equatorial plane, the second (and major) in the lateral plane, the third (and intermediate) in the sagittal plane. Therefore we have before us the same "trizonal shell" as in the important genus Trizonium among the Pylonida. But whilst in Trizonium, as in all Pylonida, the four gates between the girdles remain open, here in Larnacilla they become perfectly closed by lattice-work.
The formation of the typical "Larnacilla-shell" begins with a simple, spherical or lentelliptical lattice-shell, from both sides of which arise two latticed "lateral wings" opposite on the poles of the transverse axis. These two wings are comparable to the lateral chambers of Tholartus (among the Tholonida), but differ by two large openings. Each wing is a short cylindrical tube with latticed wall, open at both ends; the axis of the tube (going through the centre of the open ends) is parallel to the principal axis of the whole shell (and of the central chamber). Therefore both wings form together a transverse ring, the middle of which encloses the central chamber. The distal parts of both wings grow towards the poles of the principal axis; if they became united here, the second (lateral) girdle would be complete. Between it and the first girdle four open gates remain ("Tetrapyle"); but these become overgrown by the third or sagittal girdle, and at last the gates between this and the two other girdles become closed by lattice-work. This perfect fenestration of the trizonal cortical shell, and the complete closing of the gates between the girdles by network, is the only difference between Trizonium and Larnacilla.
In Larnacilla and in the nearly allied Larnacidium (only differing by radial spines on the surface) the "trizonal shell" is an external or "cortical shell," enclosing the central capsule, the interior of which only contains the simple central chamber and the jointed proximal parts of both lateral wings. In the other genera of Larnacida (by proceeding growth) this trizonal Larnacilla-shell becomes enclosed by the growing central capsule and is now only a "medullary shell," whilst on the outside of the central capsule in the same manner is developed an outer cortical shell (Larnacalpis, Larnacantha); and perhaps the same process may be repeated. But sometimes also this cortical shell becomes doubled by a simple envelop of network (Larnacoma). In the Larnacospongida the cortical shell is composed of a spongy framework (corresponding to the Spongolarcida in the foregoing family).
The lattice-work of the Larnacida is commonly irregular (as in most other Larcoidea), and its pores have little signification for the different species. On the outer surface often arise radial spines, symmetrically disposed either in dimensive planes or in diagonal planes.
The central capsule is constantly a true lentellipsis or a "triaxial ellipsoid," characterised by three halving, elliptical dimensive planes, perpendicular one to another. It bears a different relation to the skeleton in the two subfamilies of Larnacida. In the first subfamily, the Larnacillida (Larnacilla, Larnacidium), the central capsule encloses only the simple spherical or lentelliptical central chamber ("simple medullary shell"), and is enveloped by the trizonal cortical shell. In the other subfamily, the Larnacalpida (Larnacalpis, Larnacospongus, &c.), this trizonal "Larnacilla-shell" becomes enclosed by the overgrowing central capsule, which now becomes enveloped by an external, latticed or spongy, lentelliptical "cortical shell."
Synopsis of the Genera of Larnacida.
II. Subfamily Larnacillida. Medullary shell simple, spherical or subspherical. Cortical shell lentelliptical, trizonal; between them four gates. |  | Shell without radial spines, | 272. Larnacilla. | ||||
| Shell with radial spines, | 273. Larnacidium. | ||||||
II. Subfamily Larnacalpida. Medullary shell Larnacilla-shaped, double; the inner spherical or subspherical, the outer lentelliptical and trizonal. |  | Cortical shell simple or double, but not spongy. |  | Cortical shell simple. |  | Without radial spines, | 274. Larnacalpis. |
| With radial spines, | 275. Larnacantha. | ||||||
| Cortical shell double, without radial spines, | 276. Larnacoma. | ||||||
| Cortical shell simple or double, wholly or partly spongy. | | Shell without radial spines, | 277. Larnacospongus. | ||||
| Shell with radial spines, | 278. Larnacostupa. | ||||||
| I. Subfamily Larnacillida. Medullary shell simple, spherical or subspherical. Cortical shell lentelliptical, trizonal; between them four gates. | ||||||||
| Shell without radial spines, | ||||||||
| 272. Larnacilla. | ||||||||
| Shell with radial spines, | ||||||||
| 273. Larnacidium. | ||||||||
| II. Subfamily Larnacalpida. Medullary shell Larnacilla-shaped, double; the inner spherical or subspherical, the outer lentelliptical and trizonal. | ||||||||
| Cortical shell simple or double, but not spongy. | ||||||||
| Cortical shell simple. | ||||||||
| Without radial spines, | ||||||||
| 274. Larnacalpis. | ||||||||
| With radial spines, | ||||||||
| 275. Larnacantha. | ||||||||
| Cortical shell double, without radial spines, | ||||||||
| 276. Larnacoma. | ||||||||
| Cortical shell simple or double, wholly or partly spongy. | ||||||||
| Shell without radial spines, | ||||||||
| 277. Larnacospongus. | ||||||||
| Shell with radial spines, | ||||||||
| 278. Larnacostupa. | ||||||||