/ 



FISSURELLA. 



muscular impression surrounds the inside of the slieUj, 

 not far from the edge; it is wider at its sides towards the 

 front than any other part, but the anterior part itself is 

 generally very narrow. The outside of the shell is gene- 

 rally marked by striae or grooves^ radiating from the 

 vertex to the outer margin ; these are mostly decussated 

 by the lines of growth. The inner margin is mostly cre- 

 nulated. 



The animal of the Fissurella is very nearly related to 

 that of Emarginula, as the shell is to the Emarginula 

 itself; the fissure in the anterior margin of the latter 

 serving for the same purposes as the perforation in the 

 vertex of the former. One difference, however, is pecu- 

 liarly observable, which is that in Emarginula, the vertex 

 is directed posteriorly, contrariwise to that of Fissurella; 

 for Lamarck is mistaken in speaking of the notch or 

 fissure in the edge of Emarginula as posterior. 



Of this marine Genus Lamarck describes but 20 spe- 

 cies, perhaps not half the number at present known, 

 some of which are very elegant, and one or two deserving 

 of particular notice; for instance, the F. Pustida and 

 macroschisma. The F, GrcEca and two or three minute 

 species are found on our coasts ; by far the greater num- 

 ber of species known are recent, they are thickish and 

 strong shells, some delicately cancellated on the outside, 

 others prettily radiated with alternating dark and light 

 colours. A few fossil species are found in the truly ma- 

 rine formations above the chalk. 



In our plate we have given representations of the 

 following species : 



Fig^. 1. Fissurella pictay Lam. 



2. • — crassa. 



3. Pustula. 



4. calyctilata, Noh. 



5. ■ macroschisma. F. hiantula^ Xjum^ 



Q, ^-—..^ fascicularis? Lam. 



