a new PalUohranchiate Genus and Sjjecies, 111 



Type species, Agulhasia Davidsonu^, mihi. 



Diagnosis. — Inequivalve : large valve incurved behind, 

 flatly convex, with sharply turned-down sides, and an ill-de- 

 fined broad shallow mesial depression; small valve slightly 

 and evenly convex ; both valves finely and numerously ribbed, 

 with their margins strongly crenulated ; marginal outline 

 wedge-shaped, rather square anteally, and acutely pointed pos- 

 teally; beak produced, solid, and well incurved at its point; 

 area in the form of an acute isosceles triangle, somewhat con- 

 vex and well defined; deltidium long and narrow — closed, 

 except at the cardinal termination, by the internal plate ; fora- 

 men incomplete and notch-like ; looj) attached by two crura 

 originating close to the teeth, and projecting about one-third 

 of the length of its supporting valve, strongly arched at the 

 front or reflexed portion, and furnished with rather long crural 

 spurs ; muscidar ini2:>ressions large and strongly marked ; car- 

 dinal muscular fulcrum large, with a central prominence rising 

 out of a deepish cavity ; teeth massive. Colour white. 



Agulhasia Davidsonii^ in its wedge-shaped outline and rib- 

 bing, strikingly resembles young specimens of Rhyncho- 



nella ; but the latter shell is without an area. There are 



about twenty ribs on each valve, which become obsolete at 

 the sides. Mr. Davidson's specimens are quite small (PI. XI. 

 fig. 1), the largest measuring f of an inch in length, ^ in 

 width, and y^ in depth : the thickness of their valves, espe- 

 cially in the umbonal region, and the excavated appearance of 

 the muscular scars, are points strongly favouring the view 

 that, small as they are, they represent a species in an advanced 

 stage of growth. The umbonal cavity is nearly filled up with 

 shell-substance. 



The long narrow form of the deltidium (PI. XI. fig. 7 b) 

 causes the area (fig. 7 a) to appear as if divided longitudinally 

 by a linear groove ; which part is at once striking and unique. 

 The foramen (fig. 7 c), which appears like a notch in the centre 

 of the cardinal edge of the large valve, is made entire by the 

 juxtaposition of the cardinal edge of the small valve (fig. 2). 

 The area is well defined laterally by each of the sutures (fig. 7 e) 

 which separate it from the inflexed sides (fig. 7 d) of the beak. 



The loop (figs. 5 and 6) agrees very closely in form and 

 relative size with that of Terabratula vitrea ; perhaps its crural 

 spurs are more produced. 



The tubuli appear to run in rows, and to be most numerous 



* I have mucli pleasure in dedicating this species to my esteemed col- 

 leaguo Mr. T. Davidson, with whom I have been in a measure more or 

 less associated for a great number of years in elucidating a group of shells 

 the favourite of us both. 



