MOLLUSCA. LAMELLIBRANCHIA 215 



3. Hinge, when well-developed, isodont ; teeth some- 

 times absent or imperfect. A median pit for the ligament. 

 Siphons absent. Posterior adductor only present. 



Spondylus (fig. 93 D, E). Shell irregular, with ears, attached 

 by the right valve ; surface with radiating ribs which are spiny 

 or foliaceous. Eight valve larger and more convex than the left, 

 with a triangular area. Two strong teeth in each valve, which fit 

 into corresponding sockets in the other valve ; between the teeth 

 a triangular ligament-pit ; ligament partly external. Adductor im- 

 pression large, sub-central. Jurassic to present day. Ex. S. spino- 

 sus, Chalk ; S. rarispina, Bracklesham ; S. gcederopus, Recent. 



Plicatula. Similar to Spondylus. Surface smooth, folded 

 or scaly. Without ears. Area very small. Ligament internal. 

 Adductor impression excentric. Trias to present day. Ex. P. 

 spinosa, Lias ; P. inflata, Chalk ; P. cristata, Recent. 



Anomia. Shell thin, irregular or sub-circular, attached by 

 a calcified byssus, which passes through a rounded sinus near the 

 umbo of the right valve. Right valve flattened, with a central 

 adductor impression; left valve larger, convex, with three impres- 

 sions of the byssal muscles and one of the adductor. Teeth absent. 

 Lias to present day. Ex. A. ephippium, Pliocene to present day. 



Pecten. Shell sub-circular, ovate or trigonal, closed, almost 

 equilateral, inequivalve or nearly equivalve. Surface frequently 

 with radiating ribs or striae, sometimes smooth or with concentric 

 ridges. Hinge-line straight ; with well-developed ears, with or 

 without a byssal sinus. A central, triangular pit for the internal 

 ligament. Adductor impression large, a little excentric. Carboni- 

 ferous to present day. Pecten includes a very large number of 

 species, which are grouped into sub-genera and sections, of which 

 the more important are : — JEquipecten (ex. Pecten asper, Upper 

 Greensand, P. opercularis, Pliocene) ; Amusium (ex. P. pleuronectes, 

 Recent) ; Camptonectes (ex. P. lens, Jurassic) ; Chlamys, Hinnites, 

 Xeithea (see below) ; Syncyclonema (ex. P. orbicularis, Chalk). 



Chlamys : shell ovate or trigonal, nearly equivalve, surface with 

 radial ribs. Ears large — the anterior larger than the posterior and 

 with a deep sinus for the byssus on the right valve. Trias to present 

 day. Ex. P. islandicus, Pleistocene and Recent. 



