| Fig. | 1.— | View from above, showing the petaloid ambulacra. |
| 2.— | View of the base, with the mouth. |
Micraster cor-anguinum (Snake-heart). [Lign. 107.]—Of this genus there are many species in the Chalk. This type of Spatangidæ are more or less oval, elongated, and heart-shaped, wider before than behind, with a sulcus, or furrow, in front. The shell is fragile, and composed of large polygonal plates; the tubercles small and irregularly distributed; the spines are short. The mouth is transverse, situated anteriorly, and protected by a strong projection of the odd interambulacrum, which is named the lip. The vent is terminal, and placed above the margin. There are but four ambulacra, and these are incomplete, comparatively of small extent, and situated in deep furrows. A large and new species of Micraster (M. cor-bovis, of Prof. E. Forbes), from the Sussex Chalk, is figured in Dixon's Fossils, pl. xxiv. figs. 3, 4, p. 342.
TOXASTER COMPLANATUS.
Lign. 108. Toxaster complanatus.
Greensand. Switzerland.
| Fig. | 1.— | Profile. |
| 2.— | View of the summit, showing the Vent at the side; e. | |
| 3.— | View of the base, displaying the situation of the mouth, and the union of the five ambulacra; their pores are not introduced in figs. 2 and 3. a. The narrow porous divisions of the shell, termed Ambulacra. b. Interambulacral spaces. c. Areæ, or spaces covered by the wide plates. d. The Mouth. e. The Vent, or Outlet. |
Toxaster (Spatangus) complanatus. [Lign. 108.]—In this form of Spatangus (constituting the genus Toxaster Agass.), the ambulacra are not depressed or furrowed, as in the preceding echinites, nor petaloid (leaf-shaped), as in those which M. Agassiz denominates true Spatangi, but converge to a point on the summit, as is shown in fig. 2; the external rows of pores are elongated horizontally, and form a kind of furrow. The odd ambulacrum is in a deep groove. The mouth is transverse, fig. 3, d; and at the anterior part of the inferior face there is a depression, which results from the convergence of the ambulacral areæ towards that point. The vent is in the posterior face. This species is from the Neocomian strata of France; I introduce it to illustrate the characters of several other echinites, which the French geologists suppose to be confined to the so-called Neocomian formation; but which also occur in the Upper Greensand of Blackdown.
Holaster is another genus of Spatangidæ established by M. Agassiz, for those echinites that are heart-shaped, with simple ambulacra converging towards the summit. The mouth is elongated transversely; the outlet is on the posterior face. A specimen first described in my Foss. S. D. (pl. xvii. fig. 9, 21), as Spatangus planus, is common in the Lower Chalk, and Chalk-marl, and abundant in the Firestone Malm-rock.