16 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



European Species 



O. cequdis Pander 1830 



O . callactis DsXm^n Mli 



O. calligramma Dalman 1 828 



O. eminens Pander 1830 



O.jrechi'Wyio^oxiky 1900 



O.jreija Opik 1930 



O. kuckersiana'WyiogotsV.y 1900 



O. orthambonites Verneuil 



O. oval a Pander 1830 



O.flayfairi Reed 1917 



O. rotunda Pander 1830 



O. sadewitzensis Roemer 



O . schmidti 'Wyiogonky 1900 



O . semicircularis Fsinder 1830 



O.subflicata Reed 1917 



O. tetragona Pander 1830 



O. transversa Pander 1830 



O. «/»<Jo Lindstrom 



Orthambonites crassicosta Pander 1830 



O.rotundala Pander 1830 



Discussion. — The long known genus Orthis, until 

 Hall and Clarke revised it in 1892, was the dump 

 box for all orthid-like forms. We here further re- 

 strict the genus to forms agreeing with its genotype. 

 Hence Orthis is specially distinguished externally by its 

 strong, rounded, simple costae which are covered by 

 parvicostella, and by the short curved interarea of the 

 ventral valve, which has produced a curved beak. In- 

 ternally the musculature of the ventral valve and the 

 short, blunt ( ? ) brachiophores of the dorsal valve are 

 characteristic. Orthis s. s. may be confused with 

 Nicolella, but there are certain external features of the 

 latter which will distinguish the two genera. In the 

 first place, the costas of Nicolella art subangular and 

 covered by concentric growth-lines, and parvicostella 

 have never been observed. Furthermore, Nicolella 

 has chilidial plates, but in Orthis there have been 

 noticed no modifications of either the delthyrium or 

 notothyrium. 



Orthis s. s. differs noticeably from American shells 

 referred to this genus by Hall and Clarke, which are 

 here called Hesferorthis. In contrast to the short 

 curved interarea of Orthis is the long, nearly plane 

 interarea of Hesferorthis. Along with the long inter- 

 area of the American genus go the receding dental 

 plates. In Orthis, on the other hand, the dental plates 

 are continued directly to the floor of the valve. An- 

 other more striking difference between the European 

 and American forms is the presence of a short flat 

 deltidium and chilidium in the latter. Such struc- 

 tures have not been observed either directly or indi- 

 rectly in the European species. Add to the differences 

 already mentioned the variations in the dorsal valve 

 of the European species, and it will be seen that the 

 American genus is decidedly different. The dorsal 

 interior of the American Hesferorthis is characterized 

 by long, pointed brachiophores, a cardinal process not 



uncommonly cleft antero-ventraUy, a larger muscle 

 field, and some variation in the pallial markings. 



The internal features of Orthis s. s. show relation- 

 ship to Billingsella and Eoorthis in the ventral mus- 

 culature and pallial impressions, and in the cardinalia. 

 In the pallial impressions of most of the Orthacea, the 

 two main trunks given off from the anterior ends of 

 the diductor scars are preserved. This is true in 

 Orthis s. s., but instead of the trunks diverging widely 

 as in Billingsella, they are convergent, being separated 

 only by a narrow ridge near the middle of the valve. 

 The pattern of the pallial sinus of the dorsal valve of 

 Billingsella and Eoorthis is too imperfectly known to 

 make comparison with Orthis s. s. possible. 



In the dorsal valves of Billingsella and Orthis the 

 structures of the cardinalia are very close. The brach- 

 iophores of both are blunt and short, the cardinal 

 processes are low septa, and the sockets are pits between 

 the outer face of the brachiophore and the wall of the 

 valve. The median ridge in both genera is primitive, 

 being merely a low, broad undulation corresponding 

 to a faint external sulcus. There is little to differentiate 

 the two genera on the basis of the cardinalia, except 

 the more advanced character of those of Orthis s. s. 

 There is thus an obvious relationship, more or less 

 remote, between Orthis, Billingsella, and Eoorthis. 

 The variation in the ventral pallial sinus impressions 

 and the presence of a deltidium and chilidium in Bill- 

 ingsella are, however, very fundamental differences. 



Hall and Clarke typified their restricted group 

 Orthis as the group of O. callactis Dalman. In so 

 doing they merely designated a body of shells within 

 the genus Orthis sensu lata, but did not state definitely 

 that they selected O. callactis as the type of the genus. 

 Furthermore, in their definition they obviously em- 

 ployed American shells, and in no sense did they define 

 any group typified by O. callactis Dalman. Later 

 Schuchert** selected O. calligramma Dalman as the 

 type of the genus. But before him Davidson^^ had 

 already defined the type of Orthis as O. callactis. This 

 is the first mention of a genotype and the designation 

 is definite, and must stand. 



It may be a shock to American paleontologists to 

 have their cherished conceptions of Orthis once more 

 upset, but it is clear that this genus as defined by Hall 

 and Clarke was based entirely on American shells, and 

 accordingly their "genus Orthis" properly belongs on 

 this side of the Atlantic. However, as Davidson 

 selected the genotype, a common European species, 

 much earlier, we are forced under the rules of nomen- 

 clature to accept it, and as the American species are so 

 different from Orthis s. s. we must give them a new 

 name, and for this purpose Hesferorthis, the "western 

 Orthis," seems to be most appropriate. 



"U. S. Geol. Surv., BuU. 87, 1897, p. 282. 



^^ Brit. Foss. Brach., vol. 1, Introd., 1853, p. 101. 



