86 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



certain species of Walcottella trend in the opposite direction toward 

 the Leperditian type. Neither is beheved to be a link in a true genetic 

 chain, and the latter is probably merely prophetic or suggestive of a 

 subsequent evolutionary departure from the same synthetic stock 

 that finally culminated in the Leperditiidae. In the case of Dielymella, 

 this is probabl}^ only an abortive expression of the tendency of the 

 old stock to modify in the direction of the ceratiocarid carapace, 

 since the true line of the latter had been established already in middle 

 and early Cambrian times. 



The small impressions in the umbonal part of the casts of the 

 interior, satisfactorily seen only in the genotype, are thought to be 

 comparable in a broad way to the impressions of the rostral clavicles 

 and muscles seen in the Ordovician bivalved crustacean genera 

 Technophorus and Ischyrina.^ These pelecypodlike shells are related 

 to Ribeiria and similar types of presumed Crustacea occurrmg in the 

 rocks of Beekmantown and Ozarkian ages, which in part close the 

 obvious gap between Technophorus and Dielymella. 



Considering generic characters, Dielymella is distinguished from 

 Indiana by the greater development and more prowlike shape of the 

 anterior part and the wider gape of the valves. In that genus the 

 antero-cardinal angle is generally rounded off and never beaklike, as 

 it commonly is in Dielymella. 



DIELYMELLA RECTICARDINALIS, new species 



Plate 10, Figures 3-7 



Description. — Carapace podlike, the greatest height and length 

 respectively as 7 or 8 is to 12; hinge line long, straight, terminating 

 abruptly at the truncated anterior extremity, but passing gradually 

 into the curve of the posterior extremity; antero-cardinal angle 

 obtusely beaklike, the angle of the anterior and cardinal sides approxi- 

 mately 90°; remainder of outline curved, the posterior part narrowly, 

 with the most prominent point above the mid-height, the ventral part 

 a more or less nearly regular elliptic curve. Commonly a slight 

 increase in rate of curvature is noted in the antero-ventral part. 

 Valves moderately convex, without nodes of any kind, the highest 

 point somewhat above and slightly in front of the middle; edge 

 usually simple; but some valves exhibit a narrow band along the 

 ventral margin, probably due to a delicate, raised line on the inner 

 surface and a slight marginal thickening of the inner edge; anterior 

 and dorsal slopes of casts of the interior usually increasing suddenly 

 just before reaching the edge, this fact causing a bluntness not 



» These genera have been described as pelecypoda. The doubt as to the systematic position of these 

 shells raised by Ulrich in 1894 (The Lower Silurian Lamellibranchiata of Minnesota, p. 613) has been 

 strengthened by further study, so that it is now regarded as practically settled that they are bivalved 

 Crustacea and not Pelecypoda. 



