132 MONOMYARIA. 



Genus 26.— CREN AT UL A.— ZawiarcA. 



Generic Character. — Subequivalve, flattened, somewhat 

 distorted and lamellar; without any particular opening for 

 the byssus; hinge lateral, linear, marginal, and crenated; 

 the crenulations in a regular series, callous, and hollowed 

 for the reception of the ligament. 



The distinction betwixt this genus and Pema, consists in the hinge 

 of Crenatula being composed of slightly concave caUous crenulations, 

 which receive the ligament; while in Perna it consists of parallel 

 truncated linear teeth, coiresponding and opposed to the opposite ones, 

 the ligament being inserted in their interstices only. 



Crenatula mytiloides. — The Muscle- shaped Crena- 

 tula. Plate XV. fig. 14. Oblong-ovate, oblique; the 

 base acute ; violet-coloured, with obscure radiations. In- 

 habits the Red sea. 



TRIBE II. MYTILACEA. 



Hinge with a subinternal marginal ligament, which is 

 linear, very entire, and occupies a considerable portion of 

 the anterior margin; shell rarely foliaceous, generally thin, 

 and brittle. 



Genus 27. — PINNA. — Linnceus. 



Generic Character. — Longitudinal, wedge-shaped, equi- 

 valve, gaping at the end, and pointed at the summit, with 

 the beaks straight and acute; hinge lateral and without 

 teeth; ligament marginal, linear, very long, and half internal. 



Pinna ingens. — The Huge Pinna. Plate XV. fig. 15. 

 Nearly triangular, horn-coloured, smooth, with distant folia- 

 tions; striated on one side, with rough scaly wrinkles on 

 the broader end. Twelve inches long. Inhabits the coast 

 of Britain. 



Genus 28 — MYTILUS — Linnceus. 

 Generic C/m/acfer. — Longitudinal, equi valve, with the 



