Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/79

Rh Length 1 inch, breadth 1 inch, height inch.

Locality.—Mount-Gambier Limestone.

The smaller specimens are flatter; in one the periproct is marginal, and in another supramarginal; their ambitus is blunter than that of the large specimens.

, sp. nov. Plate III. figs. 9 & 10.

The outline of the ambitus from the actinal surface is ovoid, being rounded and broad anteriorly and narrower and slightly pointed posteriorly. This outline is less evident from the abactinal surface on account of the keel which passes from the vertex posteriorly, and of the slight roundness of the test on either side of it near the ambitus. The test is thick in substance, and in general shape is rather depressed, but convex above and concave below. It is arched from the front to behind the apical system, which is slightly eccentric to the front, but is slightly flattened anteriorly, and to acertain extent laterally above the ambitus. The arched shape is not so decided posteriorly, where, near the vertex, the keel starts obliquely backwards and downwards to overhang, at about one fourth of the height, the small transverse periproct, which has a flat and shallow groove. The greatest width is just behind the antero-lateral ambulacra; and the mouth and apical system nearly correspond, the first being slightly more anterior than the other.

The apical system is elongate; and the anterior and posterior pairs of generative pores are wide apart, the ocular plates of the antero-lateral ambulacra coming well in between them (Pl. III. fig. 10). Hence the posterior; lateral ambulacra are more distant than is usual in the Cassidulidæ. The anterior generative pores are large, closer together than the posterior; and the madreporiform body is small, convex, and reaches just in front of the right ocular foramen. The posterior pairs of generative pores are wider apart and slightly smaller than the anterior, and are posterior (by the length of their own distance apart) to the ocular foramina of the antero-lateral ambulacra. The rectangular space between the anterior and posterior generative pores is slightly depressed, the madreporic body forming however an elevation in it. This space is covered with miliaries and a few small tubercles.

The ambulacra are long and narrow, being, with the exception of the anterior, which is slightly in a groove, flush with the test. The poriferous zones are lower, and are continued to the ambitus, the distance between the pairs of pores gradually increasing near the edge.

The poriferous zones are narrow, and not so wide as the inter-poriferous; and the pores of the inner rows are round and smaller than the oval and more or less elongate kinds of the outer rows. They are conjugate. The pores of the odd anterior ambulacrum are