GENERA OF THE SUBORDER ORTHOIDEA 



117 



shell features of H r mi promt rs differ from those of 

 Ort/iii s. s. only in having a spondylium. 



The dorsal interior of Hfmipronitfs is likewise dis- 

 tinctly orthoid. The brachiophores arc rather flat 

 plates, divergent and oblique to the vertical. The 

 space between the sloping outer face of the brachio- 

 phore and the wall of the valve forms the socket. The 

 brachiophores are supported and usually nearly en- 

 gulfed by adventitious shell of the notothyrial plat- 

 form, but never to the extent seen in Vrllnrrio. The 

 median ridge is rather high and extends to the middle 

 of the valve. The posterior diductors are smaller than 

 the anterior pair which are elongate and elliptical. 

 Finally, the whole interior of the valve outside the 

 muscle field is marked by elevated, irregular radiating 

 ridges, and there can be no doubt that the spaces 

 between some of these represent pallial sinuses although 

 no definite system could be discerned. 



Verneuil'" united nineteen of Pander's species under 

 Von Buch's name Orthis hem'tpronites. This is an- 

 other example of Verneuil's "lumping" attitude and 

 his failure to understand Pander's ability in species 

 making. There can be no doubt that many of 

 Pander's species of Hem'tpronites are insufficiently 

 drawn, but even so, it must be said that all of his nine- 

 teen forms can not be put into one species. There is 

 great need for a critical revision of the species occurring 

 in the "Glauconite" horizons by someone thoroughly 

 familiar with the local stratigraphy of these shells. 



Opik has referred Pander's species H. maximus to 

 the genus Clitambonites. A large specimen in the 

 Schuchert Collection (Cat. No. 365), however, agrees 

 with Pander's figure (pi. 16, B, fig. 5) except in the 

 arching of the ventral palintrope, and it shows all the 

 characteristics of /f<?wi/>ron/Ve^ (see pi. 8, figs. 12, 13). 

 We believe, therefore, that Opik has not figured H. 

 maxima of Pander but actually has a new species not 

 referable to Hemipronites but with a shell surface 

 which relates it to Vellamo squamata. 



Genus PAHLENELLA Schuchert and Cooper 

 1931 



(After Pahlen, in recognition of his fine work on 

 Orthisina) 



PI. 7, figs. 1-4 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 245. 



Genoholotype. — Orthis trigonuln {ex parte) 

 Eichwald 1840, Sil. Schicht. Esthland, I, p. 148; 

 Pahlen, Mem. Acad. Imp. Sci. St. Petersb., 7th ser., 

 vol. 24, no. 8, 1877, p. 46, pi. 4, figs. 22-24. 



Description. Exterior. — Semiclliptical, hinge- 

 line straight ; cardinal extremities acute; lateral profile 

 concavo-convex; anterior commissure sulcate; ven- 



' Geology of Russia, vol. 2, Paleontology, 1845, p. 205. 



tral valve provided with a prominent fold. Ventral 

 palintrope the longer, moderately apsaclinc; dcltidium 

 perforate. Dorsal intcrarea nearly obsolete, hyper- 

 cline. Surf.ice coarsely multicostellate. Test fibrous, 

 impunctate. 



Ventral interior. — Teeth prominent, crural fossettes 

 deep; spondylium short, supported by a low septum 

 which is not, however, produced forward beyond the 

 anterior end of the spondylium (sec pi. 7, fig. 2). 



Dorsal interior. — Notothyrial cavity nearly obsolete, 

 brachiophores as in Clitambonites and Gonamhonitrs. 

 Cardinal process, along with the median ridge and 

 notothyrial platform, fused with a sessile shield-shaped 

 plate of adventitious shell underneath the muscle field. 

 Posterior adductor scars the smaller. 



Geologic range. — Lower Ordovician (lower 

 Biii^ or Kunda formation) of western Russia. Only 

 known species the genotype. 



Discussion. — In this interesting little shell there are 

 several features that set it apart from the rest of the 

 Clitambonitidas. Externally the concavo-convex pro- 

 file is not in itself unique, but this, when combined 

 with a prominent ventral fold and a corresponding 

 dorsal sulcus, makes an aggregate of characters suffi- 

 ciently distinct for generic discrimination. Internally 

 there are additional striking variations in each valve. 

 In the ventral valve, the spondylium is supported on a 

 septum so low as to give it the appearance of being 

 sessile; the septum is furthermore very short and in 

 this is very unlike the long ones in Clitambonites. 



In the dorsal valve, the development of an elevated 

 adductor field is a convergence toward the type of 

 structure seen in Leptelloidea Jones. The disc is 

 shield-shaped and produced anteriorly into a short 

 process, while the pattern of the muscle marks does 

 not recall those of Clitambonites. 



Genus APOMATELLA Schuchert and Cooper 

 1931 



(Gr. a, without; potiia, cover) 



PL 7, figs. 6, 7, 12, 13 



Schuchert and Cooper, Amer. Jour. Sci. (5), vol. 22, 1931, 

 p. 245. 



Genoholotype. — Orthisina ingrica Pahlen 1877, 

 Mem. Acad. Imp. Sci. St. Petersb., 7th ser., vol. 24, 

 no. 8, p. 48, pi. 2, figs. 18-21. 



This name is suggested for the small Orthisina 

 ingrica Pahlen which is devoid of a deltidium and 

 chilidium. This loss of important features is a con- 

 vergence toward the Orthida: and appeared mther 

 early in this genus of the Clitambonitida;. The ventral 

 valve is subpyramidal, with a catacline or procline inter- 

 area. Internally the spondylium is short and elevated 

 upon a high septum. The interior of the dorsal valve 

 was not observed. The exterior is multicostellate. 



