144 MESOZOIC AND CBNOZOIC ECHINODERMATA OF THE UNITED STATES. 



which are nearly equal in length, closing at the ends. Peristome apparently slightly excentric 

 anteriorly. Periproct small, elliptical, transverse, supramarginal, in a shallow indentation 

 close to the posterior end of the test. 



Dimensions. — Length 32 millimeters; width 21 millimeters; height 15 millimeters. 



Description. — This species was first reported in 1851 by T. T. Bouve of Boston. The doubt 

 in regard to the generic position of Bouve's form, which Desor recognized as existing because 

 of a lack of definite knowledge in regard to the inferior surface, is partly resolved by an exami- 

 nation of the type, which though not showing a floscelle does show a cribriform median area 

 on the under surface. A valuable corroboration of this evidence is found in the fact that one 

 specimen of Cassidulus depressus, the form most nearly related to the present species, shows 

 a well-defined floscelle. The test of this species is elliptical in marginal outline, narrowing 

 noticeably toward the posterior end. The upper surface is somewhat elevated, convex, elongate- 

 subcorneal, sloping steeply to the margin, slightly less so posteriorly than on the sides and 

 anteriorly; under surface flat, meeting the sides at the margin in an acute angle; apex slightly 

 forward of the center. The general appearance of the form, as the name given by Bouve 

 implies, is much like that of a limpet shell such as those of the genus Patella. 



The ambulacral areas are narrow, narrower at the ambitus than in the petaloid region; 

 dorsal portions petaloid, the petals rather long, the anterior pair shorter than the posterior 

 pair and odd petal, which are nearly equal in length; petals closing at their extremities. The 

 details of the ambulacra and apical system are not discernible on the specimen. 



The peristome is apparently slightly eccentric anteriorly; details can not be made out. 

 The periproct is small, elliptical, transverse, supramarginal; located in a small, shallow, trans- 

 verse indentation only slightly above the margin and very close to the rather pointed, truncated, 

 posterior end of the test. 



The surface of the test is covered with numerous small tubercles, with scrobicules, larger 

 on the under surface except along a rather broad median area back of the peristome where the 

 surface is free from tubercles and more or less cribriform. 



Related forms. — No other species of Cassidulus reported from the United States bears any 

 close resemblance to Cassidulus patelliformis. It is closely related to Cassidulus depressus but 

 can easily be distinguished by its greater size, more elevated and elongated subcorneal upper 

 surface, more steeply sloping posterior face, and lower placed periproct. 



Locality. — Georgia, probably Baker County. 



Geographic horizon. — Probably upper Eocene. May be lower Oligocene. 



Collection. — Boston Society of Natural History (1558). 



Cassidulus (Rhynchopygus) depressus Twitchell, n. sp. 

 Plate LXVI, figures 4a-e, 5a-b. 



Determinative characters. — Test very small, subelliptical, upper surface much depressed, 

 convex, declining gently on the sides, the posterior end sloping more steeply than the anterior; 

 undersurface slightly concave. Apex central; apical system excentric anteriorly. Peristome 

 small, pentagonal, slightly elongate longitudinally, anteriorly excentric, with a well-defined 

 floscelle. Between the peristome and the posterior border there extends a wide, elliptical, 

 cribriform area. Periproct small, elliptical, transverse, situated rather high above the posterior 

 margin, in a shallow indentation, 



Dimensions. — Specimen A (type): Length 16 millimeters; width 12 millimeters; height 

 7 millimeters. 



Description. — This small Cassidulus, one of the smallest representatives of the genus 

 occurring in American deposits, has a test which is subelliptical in marginal outline. The 

 upper surface is much depressed, longitudinally convex, declining gently on the sides. The 

 posterior median area, between the apical system and the periproct remains at nearly the 

 same elevation from which the surface slopes steeply to the posterior margin and less steeply 

 to the anterior. The undersurface is nearly flat, or slightly concave longitudinally, meeting 

 the sides at the margin in a somewhat acute angle. The apex is central or subcentral. 



