CYTHEROPTERON. 205 



siniiated towards each extremity. Seen from above, broadly ovate or lozenge-shaped, 

 with equally tapering and sharply mucronate extremities ; greatest width situate in the 

 middle and equal to rather more than half the length. End-view almost quadrate, 

 scarcely at all tapering to the apex. Shell-surface densely and minutely punctate, on the 

 ventral sm'face longitudinally striated ; alseform processes evenly and boldly rounded and 

 but shghtly prominent. 



Length, ^th of an inch. 



Distribution. Recent. — Baffin's Bay, North Atlantic. 



Fossil. — Scotland : Dryleys, Errol. Canada. 



5. Cytheropteron Montrosiense, nov. sp. Plate VIII, figs. 28 — 36 ; Plate XIV, 



figs. 13—16. 



1868. Cytheropteron Montrosiense, C, B., and U. Ann. Mag. N. H., ser. iv, 



vol. ii, p. 33, pi. V, figs. 

 1 — 5 (figured but not 

 described). 



Valves of the female (?), as seen from the side, oblong, subrhomboidal, nearly equal 

 in height throughout, height equal to more than half the length ; anterior margin 

 broadly rounded; posterior obliquely truncate below, produced above into a broad 

 laminar projection or beak ; dorsal margin slightly convex, deeply sulcate behind 

 and at its junction with the laminar process ; ventral straight in front, convex behind ; 

 lateral protuberance very prominent, forming a large irregularly rounded projection 

 behind the middle of the ventral margin. Outline, as seen from above, rhomboidal, 

 widest behind the middle, extremities pointed, the posterior strongly mucronate. End- 

 view broadly triangular, the sides very convex; lateral surfaces marked with large 

 polygonal excavations, ventral surface longitudinally rugose. Shell of the male (?) 

 higher in front, the dorsal margin sloping steeply backwards, lateral and posterior 

 protuberances poorly developed, surface-markings smaller. 

 Length, g^th of an inch. 



It will be seen by reference to the figures that the two forms included in the 

 foregoing description differ considerably in appearance, but it will also be observed that 

 the smaller, which we suppose to be the male, would by a larger development of the 

 posterior and lateral protuberances assume a shape almost identical with that of the 

 larger form ; and as the two occur in the same localities we think it best to regard them 

 as varieties of the same species dependent probably upon sex or age. C. Montrosiense 

 has been found in the recent state in company with C. inflatum. 



The form figured in Plate XIV we at one time supposed to belong to a distinct 



