GIRTY, INVERTEBRATE IW88ILS FROM COLORADO 3 



Alula gen. nov. 



Shell soleniform, very transverse. Beak strongly anterior, but not terminal. 

 Upper and lower margins contracting posteriorly. Posterior outline obliquely 

 truncated. Umbonal ridge angular, with a second plication on the post-car- 

 dinal slope. Surface elegantly sculptured by fine radial costae surmounted by 

 fine, closely arranged scales or interrupted concentric lamellae. Post-cardinal 

 slope without radial costse, but with similar squamose ornamentation. On the 

 interior, the right valve has a single long, plate-like anterior tooth, a posterior 

 tooth of similar character, with possibly a small rounded tooth at the umbo. 

 Corresponding structures appear to be developed in the left valve. A large 

 anterior scar is indicated. 



Type, Alula squamulifera. 



In a general way, these shells suggest a very transverse type of Paral- 

 lelodon, and I believe that they belong to the same family, though clearly 

 representing a distinct genus. In configuration, they differ from Paral- 

 lelodon in contracting posteriorly instead of anteriorly ; in not having the 

 angular and projecting anterior extremity, and in possessing a second 

 plication between the umbonal ridge and the cardinal border. Interiorly, 

 they differ in having a single posterior tooth ; in having a linear anterior 

 tooth similar to the posterior one, and probably in lacking the flexuoiis 

 dental arrangement at the umbones. A certain resemblance to some spe- 

 cies of Pleurophorus exists in the angular umbonal ridge and the post- 

 umbonal fold, but the Pleurophori are not radially striated, and, while 

 they have a similar posterior tooth, the remainder of the dentition is 

 quite different. 



It is not certain that any American species other than the type can be 

 referred to this genus, but, if so, they are probably to be found among 

 the forms which I provisionally included under Pleurophorella. A re- 

 semblance to Alula squamulifera, more or less marked, is found in P. 

 geinitzi, P. gilberti, P. lanceolata and Allerisma (Pleurophorella?) re- 

 flexum. Of these, the most similar is P. gilberti. Typical Pleurophorella, 

 as exemplified by P. papillosa, is probably safely distinct, although its 

 internal characters are as yet unknown, because of the deeply introverted 

 lunule and the escutcheon, both characters apparently wanting to Alula, 

 and because of the absence of radiating costse in the sculpture, although a 

 somewhat similar feature exists in the characteristic papillse, which show 

 a tendency to radial arrangement. There is, however, scarcely any com- 

 parison in this item of sculpture. As a provisional arrangement, I am 

 removing to the present genus A. gilberti, A. geinitzi and A.? lanceolata. 

 Allerisma reflexum, in spite of a general resemblance to this series of 

 forms, probably has quite different, although indeterminate, relations, 

 distinctly not with typical Allerisma. As a result of a better knowledge 



