26 CRETACEOUS PELECYPODA 



a. Siih.famUi/,— GASTROCH^NIN^. 



Both valves lying free in the tuhe, the living animal covering the former by its 

 mantle, either partially or entu'ely. 



1. (Spe^^Zerm, Try on, 18G1, (vide Proc. Phil. Acad. Nat. Sc). Valves equal, 

 widely gaping in front and below, generally with a small projection of the margin 

 internally below the umbones ; a more or less separated, triangular area runs from 

 the beaks obliquely towards the posterior end ; ligament strong ; tube clavifoi-m. — 

 I consider those forms of JRocellaria; {B. mytiloides, Lam., and oth.), which Tryon 

 separated under the above (sub-generic) name, as intermediate between the Fholadidm 

 and the Oastroch^nid^. They have entirely the form of Rocellaria, l)ut the pro- 

 cess below the beaks is generally somewhat distinct, and there is a line running 

 from the apex obliquely towards the posterior periphery, much as in some species 

 of Zirphcca which, as I have previously mentioned, also indicates a passage from the 

 former to the present family, because it does not possess any accessory valves. 



Species belonging to the type of the recent Sp. myliloides, Lam., or Sp. rostrata. 

 Sow., are known already in the cretaceous period, as will be seen in my subsequent 

 list of the cretaceous species of Gastrocemnidm, and it is probable that they will 

 also be found in older formations. 



2. Bocellaria, El. de Bell., 1802, (H. and A. Adams, Gen. ii, p. 33.5). Valves 

 equal, widely gaping in front and below ; surface of the shell regularly striated, 

 not interrupted in the middle area ; tube claviform, rather short ; — boring in shells, 

 limestone rocks, etc. 



As regards specific forms this is the most numerous genus of the family ; the 

 species are found in all seas, though most numerous in tropical countries where 

 coral-reefs are formed. Eossil species are known from the Trias iipwards, gradually 

 increasing in number and variety, until they reach their maximum of development 

 at the present time. 



3. Cuciirbihila, Gould, 1861, (Proc, Bost. Soc. Nat. Hist., vol. viii, p. 22). 

 Valves equal, gaping the whole length ; tube very short, Ovate, composed of cup- 

 shaped layers, involving shell-fragments, etc. ; it is usually attached by one side to 

 foreign objects. The type is Ciicurb. cymbia of Spongier. 



4. Gastrochcena, Spengler, 1780, {Fistidana auctorum). Valves sub-equal, 

 edentulous, narrow, gaping very nearly the whole of the ventral side. Tube straight, 

 closed in front, with a perforated septum behind the valves ; — boring in sand and mud. 



Tubes which in external appearance are almost identical with those of Gast7'o- 

 chcena are first met Avith in the Trias, but the occurrence of true si^ecies of this 

 genus is at present only recorded on good evidence from cretaceous deposits ; the 

 species are, however, rare, continuing mth the same scarcity through the tertiary 

 deposits, and there are living only three known. 



