486 



LAMELLIBRANCHIATA. 



genus have been described from the Lower Silurian upwards; 

 but undoubted forms do not appear till we reach the Secon- 

 dary period. Closely allied to this is the genus Macrodon 

 (fig. 348, E) of the Devonian, Carboniferous, and subsequent 

 deposits, in which the posterior teeth resemble those of Cucul- 

 Icea in being parallel with the hinge -line, while the anterior 

 teeth are oblique. 



The Pectunculi (fig. 348, c) have a nearly round and equi- 

 lateral shell, the beaks separated by a striated ligamental 

 area, the hinge-line curved, and the hinge-teeth forming a 

 semicircular row. Pectunculus is a comparatively modern 

 genus, and does not seem to have come into existence before 

 the Cretaceous period. Numerous species are known in the 

 Tertiary rocks. 



B 



Fig. 348. Types of Arcades. A, Interior of Lyrodesma Cincinnatiensis, showing the hinge, 

 enlarged three times Lower Silurian (after Hall) ; B, Interior of Tellinomya pectunculoides, 

 showing the hinge and adductor scars, enlarged twice Lower Silurian (after Hall) ; c, 

 Interior of the right valve of Pectunculus subpilosus Tertiary ; D, Interior of valve of 

 Limopsis aurita Pliocene ; E, Interior of Macrodon Hirsonensis Jurassic. 



Limopsis, ranging from the Jurassic to the Eecent period, 

 has an orbicular, but slightly oblique shell (fig. 348, D), with 

 a central triangular cartilage-pit, and a row of transverse teeth 

 on each side. Isoarca, of the Secondary rocks, again, more 

 nearly resembles Macrodon, having an elongated inequilateral 

 shell, with the beaks near the anterior end. The beaks, how- 



