396 Mr. Gwyn Jefifreys on Dredging among the Hebrides. 



Description of a new species of Montacuta, 

 MoNTACUTA TUMiDULA*, Jeffreys. 



Shell rhomboideo-oval, rather gibbous, thin, semitransparent, 

 glossy, and prismatic : sculpture, numerous and close- set 

 delicate, microscopical concentric strise : colour yellowish : 

 epidermis fine and silky : margins, on the posterior side 

 extremely short and sloping downwards, without any of the 

 angularity which characterizes M. bidentata ; in front gently 

 curved ; on the anterior side considerably expanding and 

 rounded ; on the back rising towards the anterior end : beaks 

 small, calyciform, blunt and prominent, incurved, but not 

 having any indentation below them; they are placed close to the 

 posterior side, which is the shortest and not one-sixth the size 

 of the anterior side : hinge-line rectangular, occupying about 

 one-third of the circumference : cartilage as in M. bidentata : 

 hinge-plate narrow and strong, thicker in the middle, not ex- 

 cavated so deeply as in the last-named species, and scarcely at 

 all in the right valve : teeth, in the right valve short, trian- 

 gular, slightly inclining inwards, not widely separated; in the 

 left valve long, erect, laminar, and parallel with the hinge- 

 line ; the anterior teeth are the largest in both valves : inside 

 iridescent and polished, very finely marked (more distinctly 

 on the anterior side) with slight lines which radiate from the 

 beaks: scars irregularly oblong, conspicuous. L. 0*075. 

 B. 01. 



Habitat. Muddy ground in the Minch off the north-west 

 coast of Ross-shire, in 50-60 fathoms. I there found only a 

 single dead specimen ; but twenty years ago I dredged another 

 in Skye, which I deferred noticing until I was quite satisfied of 

 its differing from M. bidentata. [Since this Report was pre- 

 sented, Mr. Dawson has found two more specimens in some 

 of the dredged sand which I had sent him.] Among the shells 

 procured by Professor Lilljeborg in Bohuslan, on the south coast 

 of Sweden, I observed two or three specimens of the present 

 species, one of which he kindly gave me. 



This shell is smaller than M. bidentata ; it may also be dis- 

 tinguished from that species by its narrower shape, being con- 

 vex instead of compressed, having a glossy surface, and by the 

 posterior side being extremely small, with almost a perpendicular 

 truncation. That side in M. bidentata is invariably squarish, 

 and more or less angulated. The teeth in the right valve of 

 M. tumidula are much smaller, and less widely separated by the 



* Somewliat swollen. 



