ARCACEA. 



713 



teeth are few and oblique, and at each end of the hinge become 

 parallel with the cardinal margin. The genus Ciicidlam, in the wide 

 sense, ranges from the Carboniferous period to the present day, but 

 it is a matter of great difficulty to separate the fossil forms of this 



genus from those belonging to A?-ca. 



The genus Cucullcea has been divided into sub-genera, of which the 

 two most important are Macrodon and Parallelodon. In the former of 

 these (fig. 583, e) the shell is greatly elongated, with a long hinge-line, 

 the portion of which in front of the beaks carries a number of short 



Fig. 583. — Types of ArcidcE and Nuculidce. a, Interior of Lyrodesma Cincinnatiensis, show- 

 ing the hinge, enlarged three times — Ordovician (after Hall) ," B, Interior of Tellinomya. fiectun- 

 culoides, showing the hinge and adductor scars, enlarged twice — Ordovician (after Hall) ; c, In- 

 terior of the right valve of Pectunculus subpilosus — Tertiary ; d, Interior of the valve of Lim- 

 opsis aurita — Pliocene ; e, Interior of Macrodon Hirso7iensis — Jurassic. 



oblique teeth, while the portion behind the beaks supports a few elon- 

 gated teeth which run nearly parallel with the cardinal margin. Forms 

 of this type are not uncommon in the Secondary rocks, but the earliest 

 representatives appear in the Devonian rocks, and a single species exists 

 at the present day. The sub-genus Parallelodon is very closely allied 

 to Macrodon^ possessing the same elongated posterior teeth directed 

 parallel to the hinge-line, but the anterior teeth are also sub-horizontal. 

 The species of this type are principally Carboniferous and Permian. 



In the genus Pectunculus (fig. 583, c), the shell is nearly round and 

 almost equilateral ; the beaks are separated by a striated ligamental 

 area of triangular form ; the hinge-line is curved ; and the hinge- 

 teeth are oblique and form 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, and there are about sixty living types 

 of the genus. 



