186 



Amongst recent species P. subconvexus is most nearly re- 

 lated to P. convexus, Quoy and Gaimard, from which it differs 

 by the absence of radial folds, and the fewer, more distant, and 

 flatter ribs ; it is, moreover, less inflated. In a lesser degree it 

 resembles P. polymorphoides, Zittel, which occurs at a lower 

 geological horizon. 



Lithodomus brevis, spec nov. 



JOithodomus, sp., Tate, Trans. Eoy. Soc, S. Australia, for 1885, 

 p. 124. 



Cylindrical, short ; umbones tumid, incurved, submarginal ; 

 somewhat attenuated posteriorly, convexedly truncated ante- 

 riorly; post-dorsal margin very slightly arched, posterior 

 margin rounded ; ventral margin perceptibly incurved ante- 

 medially, the sloping anterior end making with the ventral 

 margin an obtusely-rounded angle. 



Surface marked with a few concentric ridges. 



Dimensions. — Antero-posterior diameter, 21 ; greatest trans- 

 verse diameter at the middle, 11 ; greatest sectional diameter 

 through both valves at about half the total length, 11 milli- 

 metres. 



Locality.— In crypts excavated in the coral Plesiastrcea Vin- 

 centi, T. "Woods, occurring at Hallett's Cove, St. Vincent Gulf. 



This species is of about the same dimensions as L. n. sp. 



from Port Darwin, but the outline is not so curvated and the 

 umbones do not project, or hardly at all, beyond the anterior 

 margin. 



Mytilus Hamiltonensis, spec. nov. Plate xviii., fig. 9. 



Oblong-subtrigonal, angled, uncinate. Ornamented with 

 very numerous frequently bifurcating radial threads, which are 

 inconspicuously elevated, somewhat flatly rounded, separated 

 by narrower interspaces ; crossed by slender and close-set 

 lines of growth. 



Inner margin of valves denticulated all round. There are 

 two or three small teeth under the umbo in the right valve. 

 The anterior adductor scar is raised on a prominent boss, al- 

 most within the uinbonal cavity. The byssal insinuation is o£ 

 moderate size. 



Dimensions of a moderate-sized example : — Umbo-ventral 

 diameter, 47 ; length of hinge, 23 ; sectional diameter, 19 ; 

 length of posterior margin, 33 millimetres. 



Locality. — Upper beds at Muddy Creek (B. T. and J. Den- 

 nant !). 



This fossil has the general appearance of the living 31. hir- 

 sutus, Lamk., though slightly different in outline, the anterior 

 and posterior margins being nearly straight and parallel, 

 whilst in M. hirsutus they are concave and convex respectively ; 



