142 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



The types of O. suba-quata are preserved in the 

 American Museum of Natural History, catalogue 

 number 910. There are four specimens in the lot, 

 one of them considerably injured and too imperfect to 

 be of any value in this discussion. Another is an 

 internal mold of a small specimen which proves to be 

 a DoleroideSy probably D. fervetus, since the ventral 

 side of the specimen has a very marked sulcus. The 

 third specimen is a silicified free ventral valve, un- 

 doubtedly of the same species as the fourth specimen, 

 which is the shell figured by Hall as Conrad's type of 

 O. subcequata}''^ Unfortunately, the latter is also a 

 silicified specimen, so that it is quite impossible to deter- 

 mine the microstructure of the shell; it is small, 

 16.5 mm. wide, 14.5 mm. long, and 9 mm. thick. 

 The following remarks wiU supplement Hall's descrip- 

 tion. ''■'*' In the dorsal valve there is a median sulcus 

 which extends for about one-half the length of the 

 valve, after which it is lost and the surface of the shell 

 toward the anterior margin is quite regular, making 

 for a nearly rectimarginate anterior commissure. 

 However, a rather poorly defined fold on the ventral 

 valve extends to the anterior margin and there pro- 

 duces a slight ventral undulation of the commissure. 

 Swollen, hollow ribs are prominent on the surface of 

 the shell. There is absolutely no single characteristic 

 which will serve to set this shell apart from its numer- 

 ous closely similar associates from the same locality. 

 The ventral interior referred to previously is that of a 

 small shell which is strongly but evenly convex. The 

 dental plates are nearly obsolete but are continued 

 forward as a low ridge nearly to the anterior ends of 

 the diductor scars. The adductors are not enclosed by 

 the diductors, which are elongate. The adjustors are 

 situated at the base of the dental pjates. This mus- 

 culature is of the same type as that seen in punctate 

 shells having the same external appearance as the type 

 of O. suhcBquata. Hall and Clarke"^ referred O. subie- 

 quata to Dalmanella s. /., and in so doing admitted it 

 to the punctate division of the orthids. They also 

 refer to this species as a punctate shell on page 194. 

 Winchell and Schuchert^'" likewise stated that O. 

 sub^quata is punctate but made no reference to the 

 shell structure of any of the other forms associated 

 with it. Foerste made no allusion to shell structure 

 or any other important anatomical features. His refer- 

 ence to Pionodema as a subgenus of Dalmanella does 

 imply, however, that he considered the group to be 

 punctate. Therefore the name Pionodema is here 

 used for punctate shells having an internal structure 

 as described above. The use of the name thus is justi- 



""Pal. N. Y., vol. 1, 1847, pi. 32, fig. 2. 



^" Ibid., p. 118. In studying Hall's description the 

 reader must bear in mind that his use of the terms dorsal 

 and ventral valve is the exact reverse of the current usage. 



'" Pal. N. Y., vol. 8, pt. 1, 1892, p. 207. 



"^ Minnesota Geol. Surv., vol. 3, 1895, p. 447. 



fied on the basis of the close similarity of the punctate 

 shells referred to D. subcequata by Hall and Clarke 

 and Winchell and Schuchert, and the close similarity 

 of structure of the ventral interior (paratype) referred 

 to above with ventral interiors of shells known to be 

 punctate. 



The ventral musculature of Pionodema is quite vari- 

 able according to the development of the adjuster scars, 

 which widen the field, and according to the length and 

 width of the median ridge which bears the adductors. 

 Frequently the adductor ridge is produced forward 

 nearly to the anterior margin. In the dorsal valve the 

 chief variation is in the angle of divergence of the 

 crural apparatus and the presence or absence of the 

 curved ridges of adventitious shell that extend forward 

 from the brachiophore supports to unite with the 

 median ridge. Externally there is considerable varia- 

 tion, especially in the degree of development of the 

 fold and sulcus. 



Pionodema finds its closest relatives in Schizofhoria 

 of Silurian and later strata, and it may be looked upon 

 as the progenitor of this group. The external resem- 

 blance is no less striking than that of the interior. 

 Indeed, Schuchert in Zittel^'^ states that Pionodemas 

 "resemble small Schizofhorla but are impunctate." As 

 shown above, however, Pionodema as now restricted 

 is actually punctate and therefore falls easily and natur- 

 ally into the Schizophoria line. Internally there is the 

 adductor ridge which becomes more prominent in 

 Schizophoria, and the dorsal valve possesses crural 

 apparatus that is identical in its essentials to that of 

 Schizophoria. 



For the progenitor of Pionodema, search must be 

 made in the Chazy. Hall and Clarke state^^* that 

 Dalmanella had its "inception in the Chazy fauna" 

 with O. subcequata. This is, however, probably an 

 error, as all of the northern Chazy Pionodemas known 

 to us are impunctate and internally ally themselves 

 with Doleroides or Mimrlla. True Pionodema occurs 

 in the Stones River and this pushes the Pionodema 

 ancestor to pre-Stones River time. This ancestor is 

 therefore unknown. It may have evolved from some 

 impunctate stock, but it seems more likely that it came 

 out of some group in which punctation had already 

 been established. 



Pionodema appears to be confined mostly to Ameri- 

 can strata. Raymond^'^ has referred to this genus 

 Orthis girvanensis Davidson (emend. Reed), but with- 

 out a knowledge of the interior. In the Bohemian 

 fauna, Orthis redux Barrande has internal characters 

 that are very similar to those of Pionodema and comes 

 from a corresponding horizon (Black River) (see pi. 

 23, figs. 2, 4). 



'^^ Zittel-Eastman Text-book of Paleontology, 2d ed., 

 vol. 1, 1913, p. 382. 

 '"Op. cit., p. 207. 

 ""■ Bull. Mus. Comp. Zool., vol. 68, no. 6, 1928, p. 308. 



