698 



LAMELLIBRANCHIATA. 



Tertiary species, and, more rarely, by Cretaceous and Jurassic 

 types. The Limanomia of the Devonian rocks is, however, very 

 closely related to Anomia proper, from which it differs in the 

 pointed shape and radial striation of the valves, and in the fact 

 that the byssal foramen is trigonal in form. 



The extinct genus Placunopsis, ranging from the Carboniferous Lime- 

 stone to the Jurassic, resembles Anomia in many respects, but the right 

 valve is not perforated ; the ligament is lodged in a transverse, sub- 

 marginal groove ; and the adductor impression is large and subcentral. 

 The genus Placuna comprises flattened and cake-like, orbicular shells, 

 which are unattached to foreign bodies. The elastic ligament is sup- 

 ported by two diverging ridges in the right valve, with corresponding 

 grooves in the left valve. The typical forms of the genus are recent, but 

 types of no more than sub-generic value occur in the Tertiary rocks. 

 The Jurassic genus Hypotrema is intermediate between Anomia and 

 Placuna, since it possesses a byssal foramen which is completely closed 

 in the adult. 



Order II. Pectinacea. 



In this order, the mantle-lobes are completely free ; there is a 

 single adductor only; there are no siphons; there are two branchiae on 

 each side of the body ; and the foot is rudimentary and often bys- 

 siferous. The shell-structure is tubulated or lamellated, but without 

 a proper prismatic layer. The shell is inequivalve or sub-equivalve ; 

 the ligament is lodged in a pit between the beaks ; the hinge is 

 generally toothed ; and the pallial line is entire. This order con- 

 tains the three principal families of the Spondylidce, Limida?, and 

 Pectinidaz, all the recent forms of which are marine. 



Family i. Spondylid^e. — In this family the shell is inequivalve, 



Fig- 563. — Sfiondylus spinosus. Chalk. 



and is attached to foreign bodies by the beak of the right valve, 

 w r hich is larger than the left. The hinge of each valve has two 

 teeth, between which is the cartilage-pit. The adductor impression 



