MYTILACEA. 



709 



stated to range from the Silurian to the Permian, but it is princi- 

 pally characteristic of the Carboniferous and Permian rocks. 

 Though the shell of Myalina resembles that of the Mytilidce in 

 form, the shell-structure agrees with that of the Aviculidce in the 

 presence of a well-developed external prismatic layer. 



We pass naturally from the Aviculidce to the Mytilidce through 

 the genus Pinna and its allies, which have often been regarded as 

 constituting a special family (Pinnidce). In Pinna, the type-genus 



a B 



Fig. 579. — a, Pinna flabelliformis, the hinder part of the shell being broken away, reduced in 

 size ; b, Aviculopinna membranacea. From the Carboniferous Limestone of Belgium. (After 

 De Koninck.) 



of this group, the shell (fig. 579) is equivalve and wedge-shaped; 

 the beaks are placed quite at the anterior end of the shell, and the 

 posterior end is truncated and gaping. The hinge is toothless, and 

 there is an elongated ligamental groove. The shell consists almost 

 entirely of the external prismatic layer, the inner nacreous layer 

 being of extreme tenuity. The earliest types of Pinna appear in 

 the Devonian rocks, and some of these have been separated by 

 Hall to form the genus Palceopi?ina, on the ground of the convexity 



