152 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



thickened margins similar to those occasionally seen 

 in Devonian Schizophoria are not infrequent in 

 Linoporella. 



Linoforella is represented by a single species in the 

 Gotland of Sweden and by a closely related form in 

 the Niagaran of the United States. Reed^" states that 

 Orthis ■polygramma fentlandica Davidson has pits 

 between the costellae as in Linoporella, but all of the 

 figures indicated by him display neither the external 

 outline and profile nor the internal characters of the 

 genus under discussion. Hence we feel that this species 

 can not be placed in association with Linoporella. 

 Orthis turgida McCoy, on the other hand, has the 

 external outline and profile and most of the internal 

 features of the genus we are here proposing. David- 

 son's figures^^* show the cruralium exactly as it is in 

 Linoporella, and in the ventral valve (fig. 20a) the 

 muscle area is somewhat more expanded than is typical 

 in our genus. In the descriptions and figures, how- 

 ever, there is no mention of the external pits so charac- 

 teristic of L. punctata. However, we refer O. turgida 

 to Linoporella with a query because of the close similar- 

 ity of internal structure, which we consider of more 

 generic importance than the external ornamentation. 



Discussion. — Orthotropia has been well figured 

 but never has been adequately described. Since the 

 genus was made known it has been erroneously classi- 

 fied among the Pentameracea, with which it has no 

 relationships. Orthotropia appears to be most closely 

 related to Linoporella, having in common with it a 

 pseudospondylium, a cruralium, and a simple orthoid 

 cardinal process (so far as could be determined from 

 the specimens studied). 



The only known specimens of Orthotropia are in 

 the form of internal molds preserved in a hard dolo- 

 mite. This is the type of specimen that was available 

 to Hall and Clarke and also the kind preserved in the 

 Teller Collection at the National Museum. The ex- 

 ternal sculpture and the shell substance are therefore 

 unknown. This being the case, it has been impossible 

 to determine the exact relationships of Orthotropia to 

 Linoporella. Should the shell substance prove to be 

 punctate and the surface to have the characteristic 

 linoporellid pores, our genus must become a synonym 

 of Orthotropia. However, in the absence of this im- 

 portant information it has seemed best to designate the 

 group of Orthis punctata as a new genus. 



Genus ORTHOTROPIA Hall and Clarke 1894 



Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1894, pi. 84, 

 figs. 3-7. 



Genoholotype. — O. dolomitica Hall and Clarke 

 1894. 



Description. Exterior. — Shell rather small, 

 hinge-line straight, narrow; cardinal extremities ob- 

 tuse. Lateral profile unequally biconvex, the ventral 

 valve having the greater convexity. Anterior com- 

 missure faintly uniplicate. Ventral interarea long, 

 apsacline; beak curved. Delthyrium probably open. 

 Dorsal interarea short, anacline; notothyrium open. 

 Nature of shell substance and external surface 

 unknown. 



Ventral interior. — Dental plates prominent, discrete. 

 Muscle field elevated at the front end by a callosity 

 which forms a prominent pseudospondylium. Diduc- 

 tor tracks narrow; adductor field narrow, elevated 

 on a low ridge in old shells. 



Dorsal interior. — Notothyrial cavity deep. Brachio- 

 phore plates convergent, forming a cruralium with the 

 median septum, which is thin. Adductor field suboval. 

 Cardinal process a simple ridge, possibly with expanded 

 myophore. 



Geologic range. — Silurian (Racine dolomite). 

 The genotype is the only known species. 



'"Trans. Roy. Soc. Edinburgh, vol. 51, pt. 4, 1917, 

 p. 857. 



'2« Brit. Foss. Brach., vol. 3, pt. 7, pi. 32, figs. 19a, 20a. 



Family TROPIDOLEPTID^, n. fam. 

 (Tropidoleptinas Schuchert 1896) 



Genus TROPIDOLEPTUS Hall 1857 



Hall, N. Y. State Cab., 10th Rept., 1857, p. 151, figs. 



1,2; Ibid., 20th Rept., 1867, p. 280. 

 Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 1893, p. 302. 



Genoholotype. — Strophomena carinata Conrad 

 1839, 3d Ann. Rept., N. Y. Geol. Surv., p. 64. 



In this family there is a single genus, Tropidoleptus, 

 which, because of its anomalous structure, has been 

 buffeted about between the Strophomenacea and the 

 Terebratellidas of the Terebratulacea. It is not the 

 purpose here to redescribe the anatomy of Tropidolep- 

 tus; this has been done very adequately by Hall and 

 by Hall and Clarke. It is our purpose, however, to 

 make some suggestions regarding the taxonomic posi- 

 tion of the genus. The anomaly in Tropidoleptus is 

 the early appearance of what looks like a terebratelli- 

 form loop, and this character has been taken by some 

 workers as proof of relationship with the Terebrat- 

 ellidae. It was early held that the Devonian genus 

 Tropidoleptus was the forerunner of the Terebrat- 

 ellidje, neglecting the fact that the latter division did 

 not appear till Mesozoic time, an enormous time 

 interval that in itself is strong evidence against any 

 relationship between the genus under discussion and 

 the Terebratellidse. 



In the 1913 edition of Zittel-Eastman, Tropidolep- 

 tus was placed in association with the Strophomenacea, 



