504 LAMELLIBRANCHIATA. 



of the most beautiful examples of the class. They commence 

 in the Oolitic rocks, are abundant in the Tertiaries, and have 

 attained their maximum at the present day. 



The Veneridce -have been divided into a number of closely 

 allied genera and sub -genera, which, for the most part, 

 are distinguishable only by minute characters. The most 

 important fossil forms belong to the related types Venus and 

 Cytherea, which begin in the Jurassic, are more abundant 

 in the Cretaceous, and undergo a great development in 

 the Tertiary rocks. Artemis (Dosinia) has an orbicular, 

 compressed, and concentrically -striated shell, with a deep 

 "lunule," and appears to be first clearly represented in the 

 Cretaceous. Tapes, with various Tertiary species, has an 

 oblong shell, with anterior beaks. Venerupis, also commenc- 

 ing in the Tertiary, is readily recognised by the fact that 

 the shell is radiately ribbed, and at the same time furnished 

 with concentric ridges, its general shape being oblong. Lastly, 

 Petricola, ranging from the Cretaceous to the present day, 

 has a thin shell, oval and oblique in shape, and more or less 

 ventricose. 



FAM. 15. MACTunm Shell equivalve, trigonal; hinge 

 with two diverging cardinal teeth, and usually with anterior 

 and posterior lateral teeth. Ligament typically internal, and 

 contained in a deep triangular pit. Mantle more or less open 

 in front ; siphons united, with fringed orifices ; foot com- 

 pressed. The only two genera of any importance as fossils 

 are Mactra and Lutraria, both of which live buried in sand or 

 mud. The Mactrce (fig. 371) have a nearly equilateral shell, 

 with a short pallial sinus, and an internal ligament contained 

 in a triangular pit. They appear to have commenced in the 

 Cretaceous, and have attained their maximum at the present 

 day. In Lutraria the shell is oblong and gaping at both 

 ends, the pallial sinus is deep, and the internal ligament is 

 supported by a prominent cartilage-plate. The genus is not 

 certainly known in rocks earlier than the Cretaceous. 



Gnathodon, ranging from the Cretaceous to the present day, 

 has a thick ventricose shell, the beaks of which are often 

 eroded ; the lateral teeth are doubled in the right valve, and 

 are transversely striated. 



