MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA . 237 



vex, compressed behind, concentrically striated, often 

 radiatingly grooved. Umboues small, prominulous, 

 somewhat incurved. Hinge toothless. Ligament elon- 

 gated, linear, dorsal, internal. Dorsal margin straight, 

 or slightly convex, anterior extremely short and rounded, 

 anterior-inferior little convex. 



Modiola differs from Mytilus in little else than the 

 slight projection of the anterior side, which in that genus 

 is wanting. The two genera pass into each other, by 

 insensible gradations, and were the species not very nu- 

 merous, might with propriety be united. 



\. Modiola harldta. Great Modiole. Horse Mussel. 



Shell obovato-oblong, very convex ; with the dorsal line 

 straight and forming a prominent angle behind, the posterior 

 extremity rounded, the frontal side very small and rounded, 

 the anterior part of the ventral margin nearly straight or 

 sinuate ; the epidermis dusky, glossy ; the surface of the shell 

 purplish, the inside white. Length about six inches, height 

 three. 



It is very abundant on the rocky coasts, in crevices, where 

 it never attains a very large size, and is often distorted and 

 crusted. 



Lidividuals often have the margin of the shell inciu:vate and 

 widely separated at the part where the byssus protrudes. 

 Young individuals are more compressed, thinner, white, with 

 a brownish or reddish-yellow epidermis, from which project 

 numerous shreds or filaments ; whence the specific name. 



All the individuals obtained by me alive, from deep water, 

 off* Aberdeen, agree in every respect with each other, and pre- 

 sent characters which, I think, render it very doubtful that 

 Mytilus Modiolus and Mytilus barbatus of Linnosus are one 

 and the same species. Up to the length of four inches, they 

 may be described thus : 



Shell ovato-oblong, with the umbones incurved, a broad 

 tumid ridge running from them to the posterior ventral mar- 

 gin, the dorsal line straight, ascending, forming a wide rounded 

 angle with the posterior dorsal ; the anterior end extremely short 

 and roinided ; the valves thin, with a gloss)^ thin concentri- 

 cally striolate epidermis; and divided into three areas; the 

 anterior chestnut-red, separated by an oblique shallow groove 

 from the oblique tumid umbonal band, which is reddish-white, 



