288 SIDNEY F. TIAinlKR. 



proximal pari of the zooecium. The arch is usually not rounded 

 as in the great majority of forms (a zooccia of S. truncata 

 excepted), but is typically transversely oblong, witli angular 

 distal corners, giving this species an aspect entirely peculiar to 

 itself. A few b zooccia occur, however, with a nearly semi- 

 circular oral arch, even in zooccia which do not precede the 

 division of a longitudinal row. The post-oral shelf is tuber- 

 cular and very deep, descending steeply to the much depressed 

 liorizontal cryptocyst, from which it is sharply separated ; it 

 is usually narrower proximally. The oral shelf of a forms a 

 semicircular arch, whose upper surface is smooth and regularly 

 concave, and there are, of course, no tooth alveoli. That of 

 B is trifoliate, its upper surface slightly concave medianly 

 and just distally to the condyles, but strongly convex above 

 in the latero-distal parts which render the arch trifoliate. 

 The shelf of b is of an entirely different character from that 

 of other species. Whilst in these the shelf is usually concave 

 above, or in any case so arranged that there is a considerable 

 cavity between it and the closed operculum, the two convex 

 parts of the oral shelf of S. alveolata stand up above the level 

 of the oral arch, and the concave surfaces of the operculum be- 

 tween the distal end of the main sclerite and the submarginal 

 sclerite actually come into contact with and are supported by 

 these convexities when the operculum is closed. The oral 

 arch is no thicker than the post-oral edge, and thus is entirely 

 different from the strong oral arch of other species. The 

 occurrence of the series of tooth-alveoli just within the very 

 thin oral arch is highly remarkable. A smaller oral shelf 

 can be made out in the figure at a deeper level than the main 

 shelf. This may be regarded as the basal edge of the thick 

 oral shelf. This edge does not project in a. The condyles 

 are strong. The cryptocyst is thick and tubercular, its hori- 

 zontal part small in b, very small in a ; the pores are of 

 medium size. The descent into the median process is very 

 steep, but without angulation. The cavity of the process is 

 deep, short and broad in a, and with a rounded, everted 

 border in b. It may, however, be much compressed, and 



