& PALEONTOLOGY 



centrically ornamented, as a rule : the two first are 

 equivalve, without curved beaks, while Inoceramus is in- 

 equivalve, and has beaks incurved. This last attains 

 great size and thickness in the Upper Cretaceous, just 

 before extinction, showing the same senile characters as 

 Hippopodium or some brachiopods growth of the shell- 

 margin being quite out of proportion to actual increase 

 of length. Many other genera might be named. 



2. Anomiacea. Inequivalve; monomyarian ; intern- 

 ally nacreous; without ears; byssal notch (at least in 

 early life) converted into a perforation (which may after- 

 wards close up) ; no hinge-teeth ; ligament amphidetic, 

 internal. 



Anomia (Jur.-Rec.) with the above characters is occa- 

 sionally found fossil. Placunopsis (Jur.) resembles Anomia, 

 but the right valve is not perforate and is fixed by 

 cementation. 



3. Pectinacea. Usually inequivalve ; monomyarian ; 

 interior lamellar, sub-nacreous ; with thin external 

 ligament and thicker resilium, both amphidetic ; teeth on 

 either side of resilium, well-developed or vestigial; 

 usually with ears, sometimes with a byssal notch. Orna- 

 ment usually radial. Some genera fixed by byssus or 

 cementation, others active swimmers. Silurian to Recent. 



These shells resemble the Pteriacea in (a), being 

 usually inequivalve, (b) usually having the hinge 

 lengthened by " ears," (c) having an amphidetic area and 

 ligament, and (d) having a byssus, and consequently in 

 many cases a notch under the right anterior ear. On the 

 other hand, the ligament tends to sink into the position 

 of a resilium, between the teeth ; the anterior adductor is 

 entirely lost, making the shell monomyarian ; and the 

 internal layer is sub-nacreous. The chief genera are 

 Chlamys (Trias.-Rec.), both valves rather flat, with good 

 byssal notch (Fig. 24, e) ; Pecten (Cret.-Rec.), with very 



