No.iavi. NEW CARBONIFEROUS MOLLUSKS-aiRTY. 729 



PLEUROPHORELLA PAPILLOSA, new species. 

 Plate XLV, figs. 4, 5, 6; Plate XLVI, fig. 5. 



In general appearance this form much resembles species of the (^renus 

 Pleurophoru^. The size is somewhat al)()ve the avorao-c. tlic shupc 

 transversel}^ elongate, with the width al)out twice the height. Tlie 

 point of the umbo is situated but a short distance posterior to (lie front 

 end of the shell. The upper and lower margins are suljpariillcl. The 

 hinge line is straight and occupies about two-thirds the entire width. 

 The lower margin is gently convex, turning upward rather strongly at 

 the anterior end, the outline of which is truncated for about half the 

 height. The posterior end is also truncated l)y an oi)lique line, mak- 

 ing an obtuse angle with the upper and an acute angle with the lower 

 margin. In perfect specimens the posterior truncation is itself more 

 or less complex or interrupted b}^ the development of ribs upon 

 the shell. The posterior outline is not normall}^ strongly oblique, 

 but in old specimens, through l)eing worn or broken, it merges more 

 or less with the cardinal oiitline and extends strongly l)a('kward, mak- 

 ing the inferior-posterior angle sharply rounded. The convexit}'^ is 

 considerable, and is the same in both valves. ^ It is greatest toward the 

 front, diminishing markedly toward the posterior end. The antei ior 

 end is strongly concave, forming what may be termed an introverted 

 lunule. The junction of the retreating and advancing portions of the 

 shell is acutely angular. The introversion of the antei-ior end pro- 

 duces a more or less distinct truncation of the outline. Extending 

 diagonally from the beak to the inferior-posterior angle is a well-marked 

 ridge or angidation, and a second fainter ridge runs midway between 

 it and the hinge line. Here again the shell is intlexed, forming a shar])Iy 

 defined elongate area upon both valves. These areas in the two vahes 

 are nearly horizontal, making together an obtuse angle, which opens 

 outward, and, doubtless, accoiumodated a large external ligament. 



The surface is marked bv fine concentric stria? and large low plica- 

 tions, the whole l)eing minutely and elegantly papillose. The con- 

 centric markings, especiall}^ the more prominent ones, become more or 

 less suddenly and completely obsolete at the umbonal ridge. The shell 

 is thin, but nevertheless seems to consist of two layers. The outer, 

 which carries the papillose surface ornamentation, sometimes exfoliates 

 or wears off, leaving the underlying portion smooth Init for the con- 

 centric striic and wrinkles. 



The dentition and pallial and nuiscular markings are unknown. One 

 specimen, it is true, seems to show a single dental socket under the 

 beak of the left valve, but I am not quite satisfied as to the origin and 

 function of this depression. 



Cat. No. 27140, U.S.N.M. 



Locality and hoinzon.—VvnnsyXwwnww (Cisco), (iiahaiii. Young 

 County, Texas. 



