728 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



doiil)tless the lig-amont was accoinniodated. This gTOovc narrows to a 

 line at the beaks, and 1 doubt if there was any excavation at that point 

 for the resilium. As the posterior ear is considerably larger than the 

 anterior, the lioaniental groove is much more distinct than on the ante- 

 rior ear. Mention may also be made of a fact, not l)efore recorded so far 

 as I am aware, the presence of a row of small, erect spines along the 

 cardinal line. As j^et these have been oljserved only upon the anterior 

 ear, but they may possibly have existed upon the other also. In the 

 apparent absence of a median pit this form suggests AviculipecteT}., but 

 there is no hinge plate, and the ligamental furrow, in contrast to the 

 hinge plate of either Avicul'qjecten or Linupecteii^ narrows toward the 

 umbones and widens toward the extremities of the wings. The car- 

 dinal and superficial characters of this form, and the shell structure, 

 if it has but one layer, and especially if it is prismatic, as Meek believed, 

 satisfactorily^ discriminate the genus Acantliopecten . 



PLEUROPHORELLA, new genus. « 



This term is proposed primarily for a new species of pelecypod shell 

 from the Pennsylvanian rocks of Texas, but to the same group prob- 

 ably belong several species already in the literature. While the geno- 

 type, though possessing many characters in common with King's genus 

 Allerisma^ has several striking peculiarities, the other allied forms to 

 a certain extent bridge over these differences. The specific description 

 of Pleuropliorella i)aplUosa., and the discussion following it, will give 

 the characters of these shells in more detail, but it seems proper to 

 indicate at this place the most important features of the genus, which 

 are external, those of the interior being unknown. The shape is 

 transversely elongate, subrectangular; the hinge line long; the valves 

 probably in contact throughout. The lunule and escutcheon are 

 sharply defined, the former more or less strongl3^ concave. The shell 

 is thin, the superior-posterior portion with a few radial costoe, the 

 remainder marked by concentric plications, which die out more or less 

 completely at the umbonal ridge. The surface is granulose or papil- 

 lose, the granules tending to an arrangement in radial lines, and some- 

 times connected into lira3. The chief difference between these shells 

 and those grouped under Allerisma which can be at present pointed 

 out are the more sharpl}?^ defined and more strongly depressed lunule 

 and the presence of costffi, the development of which results in a trun- 

 cation of the posterior outline. 



Genotype. — Pleurojyhorella papillosa. 



« From Pleurophorus, a Pelecypod genus. 



