164 MOLLUSCA FROM THE CRAG. 



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. EMARGINULA FISSURA. Linn. Tab. XVIIT, fig. 3, a b. 

 PATELLA FISSURA. Linn. Syst. Nat. p. 1261. 



Mailer. Zool. Dan. t. 24, fig. 1-3, 1773. 

 EMARGINULA RETICULATA. J. Sow. Min. Conch, t. 74, 1815. 



Nyst. Nouv. rech. Coq. foss. Prov. d'Anv. p. 16. 



FISSURA. Dujard. Me"m. 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, costettatd; costellis longitudinalibus, striisque transversis 

 cancettatd ; vertice recurvo; aperturd oUongo-ovatd ; margine crenulatd. 



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

 costated, and transversely decussated ; costse 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 pumtura, is probably only an extreme 

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



