240 



fore a flattened appearance. Mesial sinus extending to the beak, moderately- 

 deep and wide, regularly rounded in tlie bottom, and, at the front, less than 

 one-fourth as deep as Avide. 



Surface of test with twelve or more low, very indistinct, rounded, radiating 

 plications on each side. These are rather more pronounced near the sinus, but 

 gradually disappear towards the sides. 



The impressions of the dental plates, iu the interior mould, start quite near 

 together at the beak, and extend, parallel with and exterior to, the diverging 

 margins of the sinus, nearly or quite to the front of the valve, the space be- 

 tween the dental plates and the margins of the sinus including one or two 

 plications. In one specimen, Plate VIII, fig. 21, one plication is included in 

 this way on one side, and two on the other. 



The type specimen, a ventral valve, is 17 m. m. long, 34 m. m. broad and 

 about 5 m. m. high. 



This species is based on more or less perfect specimens of four 

 ventral yalves, of which three are internal moulds, and the fourth 

 an impression of the exterior surface. Though these agree suffi- 

 ciently well together to warrant the conclusion that they belong to 

 the same species, there are, however, some points of diflFerence be- 

 tween them. 



The surface in all the above specimens is nearly smooth, and the 

 dental plates are always long, reaching almost to the anterior mar- 

 gin of the valve. The specimen taken as the type is very regular, 

 the margins of the sinus and the bases of the dental plates are 

 straight; but in some of the other specimens the margins of the 

 sinus are irregular, curving more or less, and the sinus is narrower, 

 with the dental plates removed farther from its margins. These 

 variations do not seem to be produced by distortion, but they might 

 be due to irregular internal thickening. This question, together 

 with that of the thickness of the test, cannot be determined from 

 the present condition of the casts. 



In shape this species somewhat resembles S. Pedroana, with 

 which it is associated at Erere ; but it is easily distinguished from 

 that species by the very long dental plates and the nearly smooth 

 surface. On comparing it with those varieties of S. disjunda, Sow., 

 which have long dental plates, as represented by Prof. Hall ; Pal. 

 X. Y., Vol. IV, PI. 42, Fig. 17, and PI. G3, Fig. 14, there is seen to 

 be a great resemblance, but all the specimens of S. Elizae, which 

 show plications on the sides have a perfectly smooth sinus, and this 

 is the case in the external as well as in the internal moulds. 



