474 



L AMELLIBR ANCHI ATA . 



genera (Aviculopecten and Pernopecten more especially). The 

 genus, however, is largely represented both in the Secondary 

 and Tertiary deposits, and exists under numerous and varied 

 forms at the present day. Closely allied to Pecten is the genus 

 Hinnites, in which, however, the shell is attached, during 

 the later stages of its life, to foreign bodies by the substance 

 of the right valve. It still exists, and is represented by 

 various Secondary and Tertiary species. 



In the genus Pernopecten, of the Devonian and Carbonifer- 

 ous, we have a group of Scallops, with small, nearly equal, 

 ears, the central triangular cartilage-pit being flanked by a 

 row of smaller pits on each side (thus approaching Perna), 

 and the surface being nearly or quite devoid of radiating 

 ridges. 



A much more important Palaeozoic genus is Aviculopecten 

 (fig. 330), which, as its name implies, affords a transitional 



link between the present 

 family and that of the 

 Aviculidce. The shell in 

 this genus has the general 

 form and aspect of that of 

 Pecten itself ; but the an- 

 terior ear is flattened, and 

 smaller than the posterior 

 one. There is a byssal 

 notch beneath the anterior 

 ear ; but there is no me- 

 dian cartilage-pit, and the 

 ligament is confined to a 

 narrow facet along the 

 hinge - margin. The muscular impression and pallial line 

 are as in Pecten. The species of Aviculopecten are dis- 

 tributed between the Devonian and Permian, but they are 

 most characteristic of the Carboniferous rocks, in which they 

 are extremely abundant. In the absence of a median carti- 

 lage-pit, and the lodgment of the ligament in a groove along 

 the hinge-line, the genus approaches the Aviculidce, but its 

 shell is stated by Meek to have the corrugated and laminated 

 structure of Pecten, and not the prismatic structure of the 



Fig. 330. Internal cast of Aviculopecten. Car- 

 boniferous. (After M'Coy.) 



