TEREBRATULA. 11 



our indigenous species are very few and reducible to 

 two types, I do not wish to burden the nomenclature 

 more than can be helped, and I therefore propose to 

 adopt the genera Terebratula and Argiope only. These 

 appear to have sufficiently definite characters by which 

 one may be distinguished from the other. 



Genus I. TEREBRA'TULA*, Lhwyd. PI. I. f. 1. 



Body convex : mantle free at its outer edges. 



Shell acutely triangular: beak prominent: foramen, or 

 byssal perforation, small : hinge-line curved : skeleton consist- 

 ing of horizontally projecting blades, which are often looped. 



There are only two British species, and for these as 

 many genera have been proposed by some authors. I 

 will arrange them in sections. 



A. Shell smooth : skeleton consisting of two long blades, which 

 are not looped or connected. (Waldheimia and Macaa- 

 drevia, King.) 



1. Terebratula cra'nium^ Miiller. 



T. cranium, Mull. Zool. Dan. Prodr. p. 249, no. 3006 ; Forbes & Hanley. 

 vol. ii. p. 357, pi. lvii. f. 11. 



Body cream- colour, with a brownish tinge : mantle thin ; 

 tentacles rather short, with small brown tubercles at their 

 base : arms dark brown ; cirri rather short : peduncle short 

 and compact. 



Shell oval, with sometimes a squarish outline, convex, 

 rather thin, slightly lustrous : sculpture, smooth to the naked 

 eye, but very closely tubercled when examined with a mag- 

 nifying-power : colour white: margins compressed, often trun- 

 cate and sometimes flexuous in 'front : beak rather prominent, 

 but short, worn by rubbing against the stone or other hard 

 substance to which the shell is attached : foramen oval, in- 



* From the hole in the shell. 



t From a fancied resemblance of the shell to a human skull. 



