PECTINACEA. 



701 



appearing in the Devonian rocks, while numerous living types 

 exist. 



The genus Hinnites comprises a number of forms in which the 

 shell is free when young, but becomes in adult life attached by the 

 right valve, the shell becoming at the same time thick and irregular. 

 The left valve is free, and the hinge-line is edentulous. Numerous 

 fossil forms of Hinnites are known, the earliest appearing in the 

 Trias, and the genus still survives. 



In the genus Pecten, comprising the Scallops, the shell (fig. 566) 

 rests upon the right valve, and the beaks are furnished with ears. 

 The anterior ears are the largest and most prominent, and the shell 

 is generally furnished with ribs radiating from the umbos. There 

 is a single, median, triangular cartilage-pit. The right valve is the 

 deepest, and is notched for the byssus below the anterior ear. 

 Using the name Pectefi in its modern and restricted signification, it 

 is probable that all the Palaeozoic shells so named are really refer- 

 able to allied 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. 



The name of Entolium has been given to forms of Pecten in which the 

 shell is smooth, the hinge-line is rendered angular by the outward exten- 

 sion of the ears, and there is no byssal notch under the anterior ear of 

 the right valve. Numerous forms 

 of this type occur from the Ju- 

 rassic onwards ; but the Palae- 

 ozoic shells which have been 

 placed here are probably refer- 

 able to allied genera, such as 

 Pernopecten. In the genus Per- 

 nopecten, of the Devonian and 

 Carboniferous rocks, are com- 

 prised ancient types 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. The 

 genus Crenipecten, with a simi- 

 lar geological range to Perno- 

 pecten, has the hinge furnished 



with a row of small cartilage-pits along its entire length, no central pit 

 being present. 



Of the Palaeozoic types of the Pectinidce, the most important is 

 the large and widely distributed genus Aviadopecten, which, as its 

 name implies, affords a transitional link between the present family 



Fig. 567. — Internal cast of Aviculopccten. Car- 

 boniferous. (After M'Coy.) 



