BY ROBERT M. JOHNSTON, F.L.S. 141 



likewise in the smaller valve ; beak small and incurved. The 

 mesial fold is simple, of variable width, and flattened along its 

 upper surface ; while in the ventral valve there exists a shallow 

 : sinus, interrupted by the presence of a rounded slightly elevated 

 mesial rib. The valves are likewise ornamented by a variable 

 number of rounded, or but slightly angular, ribs ; these are 

 simple, or here or there augmented by an occasional intercalca- 

 tion. In number they vary from about 8 to 30 on each valve, 

 the larger number occurring on the most adult individuals. 

 The ribs are also at times of unequal width, even on the same 

 example ; and the entire surface of the shell is ornamened by 

 close and regular scale-like, concentric, imbricated laminae. 

 The interior of the ventral valve does not show a trace of that 

 elevated mesial septum which is always present in Spiriferina 

 cristata, Sp. octopUcata, Sp. Miinsteri, rostrata, Tessoni, and 

 other forms composing that sub-genus. The dental or rostral 

 plates in S. alata are also much smaller, and I might almost say 

 rudimentary ; the muscular impressions are likewise exactly 

 rsimilar to those peculiar to the genus Spirifera. In the dorsal 

 valve, under the extremity of the umbone, there exists a small 

 striated cardinal process or boss, but no hinge-plate, and a 

 little lower is seen the quadruple impression left by the adduc- 

 tor (PI. I., figs. 31, 32, 33a). 



Spirifera triangularis, Martin. 



Triangular, twice as wide as long, with a straight elongated 

 hinge-line, and slightly concave, nearly parallel-sided area, 

 towards the attenuated extremities of which the lateral margins 

 of each valve converge, forming acute angles with the hinge. 

 The fissure is triangular, and partly covered by a pseudo- 

 ■ deltidium. The dorsal valve is less convex than the opposite 

 one with an. elevated mesial fold which commonly assumes the 

 character of a single produced and acutely angular cuneiform 

 'ridge or rib, at times considerably prolonged beyond the frontal 

 level of the lateral portions of the valve. On either side of 

 this central ridge from 6 to 10 smaller ribs ornament the 

 lateral portions of the valve. The beak of the ventral valve is 

 narrow, produced, and incurved. A shallow mesial sinus 

 commences at the extremity of the beak, and extends to the 

 front, but at a short distance from its origin a mesial or 

 central rib originates, which becomes wider and more elevated 

 and produced as it approaches the front, and corresponds with 

 the central ridge of the dorsal valve. Seven to 11 smaller ribs 

 exist also on the lateral portions of the valve, on either side of 

 the sinus. The dimensions taken from a perfect individual have 

 produced : — 



Length, 10| ; width, 2H ; depth, 6| lines. 



