164 MOLLUSCA FROM THE CRAG. 



Lamarck in this genus ; they have recently been separated into distinct genera, 

 according to the position of this aperture. In this genus the margin is cut by the 

 fissure ; in Rimula the slit is removed into the middle of the anterior portion, leaving 

 the margin entire ; in Cemoria the aperture is in front of the vertex on the top, 

 thereby connecting it with Fissurella, from the young of which it is scarcely to be 

 distinguished. The muscular impression nearly surrounds the shell a little above 

 the margin, leaving an opening in front for the head of the animal. As the shell 

 is enlarged, this muscular mark is removed and probably covered with fresh 

 calcareous matter, as this necessarily varying impression is nowhere to be seen in 

 the upper or younger portion of the shell. The genus made its first appearance, 

 as far as is yet known, in the Oolitic period. 



1. Emargintjla fissura. Linn. Tab. XVIII, fig. 3, a — b. 

 Patella fissura. Linn. Syst. Nat. p. 1261. 



— Miilfer. Zool. Dan. t. 24, fig. 1-3, 1773. 



Emarginula. reticulata. J. Sow. Mm. Conch, t. 74, 1815. 



— — Nyst. Nouv. rech. coq. foss. Prov. d'Anv. p. 16. 



— fissura. JDujard. Mem. Soc. Geol. de France, torn, ii, p. 273, 1837. 

 _ _ Nyst. Coq. foss. de Belg. p. 350, pi. 35, fig. 6, 1844. 



— — S. Wood. Catalogue 1842, var. a, vulgaris, fig. 3, a. 



— — ... var. /3, punctura, fig. 3, b. 



— rosea. Thorpe. Brit. Mar. Conch, p. 133, fig. 79, 1844. 



E. Testa elevatd, conicd, costellatd ; costellis longitudinalibus, striisque transversis 

 cancellatd; vertice recurvo ; aperturd oblongo-ovatd ; margine crenulatd. 



Shell elevated and conical ; with the vertex slightly recurved ; longitudinally 

 costated, and transversely decussated; costee generally alternating, one large and 

 one small ; base of shell oblongo -ovate, with a crenulated margin. 



Longitudinal diameter of base, \; altitude and breadth, § of an inch. 



Locality. Cor. Crag, Sutton. 



Red Crag, passim. Recent, Britain. 



An abundant shell in both formations. In the young state the recurved vertex 

 is distinctly visible; in large specimens it is not seen. The shell is very variable in 

 its proportions, some specimens not having a height more than half the longest 

 diameter, while, in others, it is even higher than long the vertex ; in some is nearly 

 central, and in others it hangs as far back as the basal margin, and even beyond it. 

 The exterior is regularly decussated, leaving large and deep alveoli upon the 

 surface; the fissure is narrow, thickened internally, especially round the upper part, 

 and in length about one third of the height. What I had considered as a distinct 

 species, and published in my Catalogue under the name of punctura, is probably only 

 an extreme variety, with a worn and altered surface. In this shell (fig. 3, b,) the 



