Owing to the rarity of well-preserved interiors of valves in this division, our knowledge of their internal characters is still far from satisfactory. The arrangement of the muscular impressions varies greatly amongst extinct genera, but we are often able to interpret them with a considerable amount of certainty by a study of the scars and the muscles of the well-known recent Lingula (Fig. [322]). The extreme specialisation of the muscles in many of the earliest genera (e.g. Lingula) is remarkable, and points to a long but so far undiscovered ancestry in pre-Cambrian times.[429] In fossil species of Crania and Lingula the muscle-scars correspond closely with those in the living representatives of these genera. In the most highly specialised family of the Ecardines—the Trimerellidae—we meet with features of peculiar interest.[430] The muscle-scars in this family (Fig. [323], A, B) are most remarkable for the development of the so-called “crescent,” (q.r.s.) which skirts the posterior margin of both valves as a sub-cardinal impression. It is believed to be the trace of a strong post-parietal muscular wall, analogous in position to that of Lingula. The three pairs of “lateral” muscle-scars in the latter genus seem to be represented by the “terminal” (s) and “lateral” (r) scars on the crescent of the Trimerellidae. A pair of “transverse” scars (t) occurs in each valve between the “terminals” and the antero-lateral edge of the “platform” (j). “Cardinal” (v), “sub-cardinal” (w), and “umbo-lateral” (x) scars also occur. The median impression which covers the “platform” (j) consists of a central, lateral, and usually an anterior pair of scars; and the impressions of the genital organs, according to Davidson and King, lie medianly posterior to the “platform.” The “platform” itself is a more or less conspicuous central calcareous elevated area occurring in each valve, but most developed in the dorsal; in some cases it is double-chambered with tubular cavities (“platform vaults,” Fig. [323], A, B, k), in others it is more or less solid. It appears to have originated through a posterior shifting of the central muscular bands, that they might be inserted behind the liver; at the same time a deposition of shelly material, to form fulcra to work the heavy valves, took place at these points. The tunnelling-out of the platform was probably due to the continual pressure of the lobes of the liver. The division of the umbonal cavity into definite chambers in Monomerella, and to a less extent in other members of this family, appears, according to Davidson and King, to have been caused by pressure of the ovarian lobes.
In connexion with the foregoing remarks on the development of the “platform,” it may be mentioned that the paths along which the muscle-bands move, as the shell of Brachiopods increases in size, are marked by elongated scars, and often by shelly deposits; and when the members of a muscle-pair come into juxtaposition these shelly deposits (which act as fulcra for the muscles) combine, and by the growth of the shell form a septum, as in the case of the median septum of Lingulepis.
The Obolidae show some important features in the internal impressions. Obolella crassa (Hall) may be taken as a well-known type of the family. In this species a pair of small scars, one on each side of the pedicle-groove, lies close under the hinge line in the ventral valve. There is also a well-marked scar for the insertion of the pedicle-muscle at the end of the pedicle-groove. A pair of much elongated lateral impressions extending forward from the “cardinals” may be homologous with the “laterals” of Lingula; and the two small central scars between them may be compared with the “centrals” of Lingula which are in a somewhat similar position. In the dorsal valve of O. crassa a pair of “cardinals” is found, and on each side of a low median rounded ridge are two small “central” scars. Indistinct “lateral” scars arise close to or in the central area, and diverge anteriorly.
Sometimes a great concentration of muscle-scars occurs round the foramen in the ventral valve, as in Siphonotreta.
As regards the minute structure and composition of the shell in the Ecardines, we find that the Lingulidae and Discinidae have their shell composed of alternating layers of phosphate of lime and a corneous substance; the former layers are pierced by microscopic canals. The Craniidae have calcareous shells traversed by tubules, which divide into many fine branches near the external surface; a thin periostracum covers the exterior. The Trimerellidae have heavy thick calcareous shells, for which they required the previously-described elaborate arrangement of muscles to open and shut them.
II. TESTICARDINES
External Characters
It is to this division that the great majority of the Brachiopoda belong; and the diversity of form, of ornamentation, and of internal characters is correspondingly greater than in the Ecardines.
A transversely or longitudinally oval shape of shell is the commonest; but sometimes it is triangular, as in Rhynchonella (Fig. [327]), or bilobed, as in Pygope (= Terebratula diphya). The ventral valve is usually more convex than the dorsal, and the former may be prolonged into a tube by the accelerated growth and infolding of the anterior and lateral margins, producing a very abnormal form (Proboscidella). The external surface of the valves is frequently ornamented with more or less prominent radiating ribs; and fine concentric growth-lines are commonly shown, and may be developed into coarse ridges or wrinkles, particularly in old individuals. The members of the family Productidae are usually furnished with tubular spines, which are sometimes of great length, and served to anchor the free shells in the mud, or were twisted round Crinoid stems and similar objects.
In the ventral valve of many genera there is a median sinus, with a corresponding fold in the dorsal valve, and rarely vice versâ; sometimes the fold and sinus are double.