200 MYTILIDJE. 



rises too, far more obliquely and rapidly in front of the 

 shell than at its termination. The umbones are swollen, 

 and the beaks are terminal or nearly so, not unfrequently 

 projecting beyond the curved anterior outline. The hinder 

 end, which, from the greater projection below of the 

 produced and arcuated posterior outline, occasionally ap- 

 pears bluntly wedge-shaped, is also rounded both above 

 and below, there being no decided angulation of the hori- 

 zontal (or even ascending) dorsal margin, which, moreover, 

 rarely exceeds the length of the space that extends from its 

 cessation posteriorward. 



Three-quarters of an inch in length, and about five lines 

 in breadth, are the dimensions of rather a large individual. 



The animal of this species was carefully examined by 

 Mr. Clark in 1835, and re-examined by that excellent 

 observer last year. His notes are of such interest, that 

 we print them entire as communicated. " Animal elon- 

 gated, thick, oval, pale yellow; mantle closed in the ante- 

 rior ventral half, where it makes a considerable opening for 

 the emission of the foot ; it is then again closed and forms 

 a red purplish and flaky-white membrane, which is pro- 

 duced into a small cylindrical anal tube, grooved at the 

 base, and with four or five very minute dark cirrhi at the 

 terminus. This grooved anal siphon carries on each side 

 of it the mantle formed into small pendulous flaps of the 

 same colour as the tube, and the animal by bringing their 

 margins into contact forms together with the groove under 

 the anal tube a canal to convey water to the branchire ; 

 excepting these small puckered flaps, the margin of the 

 mantle where open is plain. The foot is white with a very 

 deep byssal groove at the point of its elbow, from whence 

 a strong byssus issues and fixes the animal to shells, 

 stones, the Ascidia mentula, &c. The anterior part of the 



