CLINTOX, LOWER IIKLDEKBKUG, CllEMUJS'G, AND NVAVEULY CJliOUPS. 441 



SYRINGOTIIYRIS RANDALLI Simpson, n. sp., Fig. 6. 



SliL'U transversely somi-elli])tical or semi-circular; vcntricosc, becoming gibbous 

 with age; k'ugth usually about one-half the width, but sometimes three-fourths; 



VIII f. 



APS. Tr. 1889. 



hinge line straight, length equal to the greatest width of the shell. Cardinal angles 

 more or less rounded, not attenuate. 



Ventral valve with a high vortical or slightly sloping cardinal area, Iroin the 

 apex of which the sides of the shell curve outward to the antcro-basal margins. 



A median sinus begins at the beak, rapidly widening as it extends forward, and 

 becoming deep, with abruptly sloping sides ; where it reaches the margin the shell is 

 produced in a conspicuous linguiform extension. 



The deltidial aperture is covered foi- about one-half its length from the beak, by 

 an arched transverse callosity or pseudo-dcltidium. The edges of this callosity unite 

 with the strong dental lamelhe, which divide the rostral portion of the shell into three 

 chambers; and from the inner posterior surface of the callosity extends the syringo- 

 thyi-al tube, which is unusually broad near its posterior extremity, but tapers rapidly 

 to an open termination, sloping into the internal cavity. This tube is split for its 

 entire length along its outer surface, and appears to have been thickened and fdlcd in 

 its posterior portion with the increasing age of the animal. 



Dorsal valve convex, greatest convexity at about one-third the length of the 

 shell from the beak ; convex to the cardinal line, becoming somewhat (lattoned at the 

 cardinal extremities; gradually curving to the lateral and basal margins. Mesial 

 fold narrow at the beak, raj/idly widening and becoming pi'ominent below; pioduccd 

 at the margin corresponding to the linguiform extension of the ventral valve. 



Surface of the valve ornamented by from forty to sixty costaj, which occur lioth 

 on the sides and the siiuis. liadiating .striae crossed by concentric stri.i', wliicii, in 

 the specimens observed, are most conspicuous on the mesial fold and sinus. There 

 arc also lines of growth, which are usiuilly the .strongest on the anterior portion of 



A. r. s. — vol,. \vi. .'}i>. 



