302 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIAtA. 



with a tliin prominent oblique marginal tooth, a sinus 

 under the umbilicus, and an adnate concave tooth, in 

 each valve. Ligament internal occupying the cavities of 

 the concave teeth. 



This genus differs from Amphidesma in having no ex- 

 ternal ligament. The species are all very small, reside 

 in sand, or mud, or among corallines, and may frequently 

 be found on the sandy beaches. 



1. Montacuta bidentata. JBidentate Montacuta. 



Shell ovate, very thin, somewhat pellucid, very inequilateral, 

 rounded at both ends, the posterior end much shorter and 

 rather narrower ; the valves rather convex, glossy, delicately 

 marked with concentric lines, whitish or tinged with brown ; 

 inner surface highly glossed, obscurely scrobiculate ; in the 

 right valve an oblique adherent deflexed tooth behind, a wide 

 sinus, an anterior lamellar tooth, the margins channelled for 

 a short space ; in the left valve a wide sinus, the margins 

 thickened and divergent. Length two-twelfths, height a 

 twelfth and a-half. 



Found by me, in July, 1842, among shell sand on the 

 beach near Aberdeen, and afterwards on dead shells and 

 corallines from deep water. Cruden Bay, Mr. Murray. 



As Montagu observes, " the hinge of this species does not 

 strictly belong to any one of the present genera in con- 

 chology ; the teeth, though large, and standing very high in 

 proportion, have not the appearance of what are usually 

 termed primary ; they are broad, flat, more than usually dis- 

 tant for central teeth, and equally diverge ; the opposite valve 

 is destitute of either primary or lateral teeth, and has only a 

 slight cavity under the beak." The hinge however is essen- 

 tially similar to that of the three following species. 



Mya bidentata. Mont. Test. Brit. 44. PI. 26. f. 5. Montacuta 

 bidentata. Turt.Brit. Biv. 60. Mya bidentata. Penn. Brit. Zool.; 

 Ed. u. iv. 166. 



2. Montacuta ollonga. Oblong Montacuta. 

 Shell oblong, rounded at both ends, little convex, with the 

 umbones nearer the larger end, the dorsal line straight, and 

 direct ; the valves very thin, transparent, glossy, very faintly 

 striated concentrically ; the teeth horizontally prominent ; the 

 colour hyaline-white, the interior highly glossy. Length two- 

 twelfths of an inch, height a twelfth. 



