702 LAMELLIBRANCIIIATA. 



and that of the Aviculidce. The shell in this genus (fig. 567) 

 has the general form and aspect of that of Pecten itself; but 

 the anterior ear is flattened, and smaller than the posterior one. 

 There is a byssal notch beneath the anterior ear ; but there is 

 no median 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 hinge-line is usually shorter than the 

 transverse diameter of the shell, and the surface is generally adorned 

 with radiating ribs. The species of Aviculopecten are distributed 

 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 cartilage-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 

 former. The genus Streblopteria of the Carboniferous and Permian 

 rocks comprises forms which agree with Aviculopecten in having the 

 ligament in a groove along the margin of the ears ; but the posterior 

 ear is hardly marked off from the hinder margin of the shell, and 

 an oblique posterior cardinal tooth is present. 



Order III. Mytilacea 

 ( = Heteromyaria, Bronn). 



In this order the mantle-lobes are free, the pallial line is entire, and 

 there are usually two adductor muscles. (In the Tertiary genus Dreis- 

 seno??iya the pallial line is sinuated ; while the recent genus Prasina 

 is monomyary.) The anterior adductor is small ; the posterior is of 

 large size. The foot is tongue-shaped and usually byssiferous. The 

 shell possesses an external, often largely developed, prismatic layer, 

 and may be inequivalve or equivalve. The ligament is generally 

 contained in several marginal grooves or in an oblique furrow ; and 

 the hinge may or may not be provided with teeth. The two prin- 

 cipal families contained in this order are the Aviculidce and Mytilidce, 

 and the recent forms are in part marine, and in part inhabitants of 

 fresh waters. 



Family i. Aviculidce. — In this family the mantle-lobes are free, 

 the foot is small and byssiferous, and the shell-structure consists of 

 an internal nacreous layer and an external prismatic layer. The 

 anterior adductor 1 is small, and leaves its impression at the base of 



1 The muscle which is here spoken of as the " anterior adductor" is sometimes 

 regarded as, in some cases, being really the anterior retractor of the foot. On 

 this view, there is no anterior adductor muscle in some of the most typical forms 

 of the Aviailidce, the animal being monomyary. 



