8AXICAVA. 133 



ad terminum elevato ; margine ventrali inflate ad terminuni 

 elevato. — Having- a tortuous appearance, the uraboes and 

 posterior end being raised, so that the dorsal margin forms a 

 depressed arch ; the ventral margin is swollen behind the 

 hiatus. 



3. RUGOSA, f. 3, Linnceus (Mytilus). — Tenuiuscula, elon- 

 gato-rhomboidea, sub-lff>vigata, ad terminos utrinque depressa, 

 antiee rotunda, postice subtruncata, umbonibus iniequalibus, 

 fere terminalibus.— -Although named ' rugosa,' it seems to be 

 generally a rather smooth shell. The ends are slightly de- 

 pressed, so that the back curves slightly upwards in the 

 middle. The umboes are nearly terminal. 



4. GrUERiNii, f. 6, Payraudeau. — S. arcUcre similis, sed 

 brevior, antiee ventricosa, margine terminali declivi ; um- 

 bonibus fere terminalibus. — The figure of this shell is not 

 sufficiently attenuated posteriorly, and therefore does not 

 represent that point of difference between it and S. arctica. 



5. PURPURASCENS, f. 7, Sowevby. — Rectiuscula, attenuata, 

 postice purpurea, oblique biangulata ; angulo inferiori rugoso, 

 subcarinato ; umbonibus ante marginem anteriorem pro- 

 ductis. — Strongly angular and keeled posteriorly, with a 

 strong purple color. Umboes in front quite terminal. 



6. SOLIDA, f. 12, Sowerhy. — Crassa, dactyloidea, insequi- 

 valvis, angnlis posticis submoniliferis ; area postica truncata, 

 antica rotundata, umbonibus terminalibus ; valva dextra 

 quam sinistra magis profunda et extensa ; margine dorsali 

 complanato. — A very peculiar date-shaped species, in which 

 the right valve is deeper than the left and overlaps it. Tlie 

 shell is thick, with the lower keel almost beaded. It is 

 truncated at the back and rounded in front, with the umboes 

 terminal. 



7. TENUIS, f. 11, Sowerhy. — Laevis, tenuis, postice atten- 

 uata, breviter truncata ; antiee subrotunda. — Thin, smooth, 

 narrow towards the back, and truncated at the end. 



8. spiNiFERA, f. 4, Soiverhy. — Parva, subquadrata, conspicue 

 biangulata, ad angulos spinosa ; margine ventrali antiee 

 coarctata; umbonibus terminalibus. — A small shell, with two 

 rows of spines placed on the posterior angles. The ventral 

 margin is curiously divided by a deep sinus. 



