Seymour Island, West Antarctic. 261 



there is a slightly raised ridge along the median line of the anterior 

 part of the valve, probably separating the muscular impressions, 

 which are not clear. 



Interior of dorsal valve : — Posterior part much thickened with 

 elevated solid cardinalia bounded on each side by deep hinge sockets 

 which converge posteriorly. As in other species of JBouchardia, 

 distinct socket ridges, hinge plates, cardinal process, etc., cannot be 

 certainly distinguished. It would perhaps be more correct to term 

 the socket walls in this case hinge teeth of the dorsal valve, for there 

 is ou each side a rounded projecting ridge which fits into the groove 

 in the bifid teeth of the ventral valve. The space between the socket 

 ridges is filled with shell matter, forming a solid platform, above 

 which rises a median boss or cardinal process, rounded anteriorly, 

 but with a small posterior tongue. From the sides of this boss two 

 narrow ridges converge to meet near the umbo. In the anterior end 

 of the solid hinge platform there are three caves entering from the 

 floor of the shell, a larger and deeper median one and two smaller 

 lateral ones, separated by two small projections. The high median 

 septum is situated anteriorly and touches the median projection from 

 the floor of the ventral valve. It rapidly lessens in height posteriorly 

 and projects into the median cave of the hinge platform without 

 uniting with the latter. On the anterior end of its elevated portion 

 it bears a small swollen boss. No sign of the descending brachial 

 arms has been observed in any of the numerous specimens examined. 



Dimensions in millimetres: — 



Length. Breadth. Thickness., 

 . Holotype .... 4-5 3 1-5 



Paratypes .... 4-5 3 2 



,,.... 5 4 1-8 



,,.... 4 3 1-5 



7'i/pe locality. — Base of main limestone, cuesta between Mt. Brown 

 and the Waipara Eiver, North Canterbury. 



Material. — A large series (several hundreds) from the type locality, 

 all from one small cave in the limestone ; a few specimens from the 

 same horizon in the Weka Pass end of the district ; a small series 

 from Flat Top Hill, Oamaru District, Otago. 



Horizon. — The main Mt. Brown limestone is near the top of the 

 sequence of Oamaruian beds of the Weka Pass and Waipara district, 

 and probably correlates with the Hutchinson Quarry beds of Oamaru. 

 It may therefore be called Upper Oamaruian. The limestone of Flat 

 Top Hill is Ototaran, i.e. Middle Oamaruian. The Middle and Upper 

 Oamaruian are by general consent Miocene. 



Zoological comparison. — Bouchardia minima is to be compared with 

 the other elongate species of the genus, from which it differs by the 

 narrowness and curvature of the hinge-line. B. antarctlca,^\\(ik'n\ln^, 

 is an elongate form with a broad, nearly straight hinge, then follow 

 with narrower and more curved hinges B. attenuata, Buckman, 

 B. rosea (Maine), and finally B. mimyna, in the order named. If the 

 development of elongation as revealed by the growth-lines is coti- 

 sidered B. vmiima must be placed before B. rosea, since the youthful 

 crowth-lines are broader in the former. 



