GENERA OF THE SUBORDER ORTHOIDEA 



95 



European Species of Pl/Esiomys 



Orthis {Dinorthis) fabellulum carrickensis Rccd 1917 



O. infijia Salter 



O. fore at a McCoy 1 846 



O. Solaris von Buch 



O. (D.) striato-costata Salter 186S 



O. \d.) suiJirisa Salter 1865 



American Species of Retrorsirostra 



Dinorthis carUyi (Hall) 1860 

 D. carleyi insolens Focrste 1909 

 D.retrorsa (Salter) 1858 



Anticosti Species of Pionorthis 



Dinorthis carUtona Twenhofel 1928 

 Orthis sola Billings 1 866 



Distinguishing characters. — Dinorthis is char- 

 acterized chiefly by its convexo-concave profile, svib- 

 quadrate and anteriorly bilobate plan of the ventral 

 muscle field, Or?/;w-type brachiophores, crenulated 

 myophore, and pallial sinuses. From Orthis s. s. it 

 differs in its contour, musculature, and pallial markings. 

 Discussion. — Dinorthis of the Dinorthidae and 

 Dolerorthis of the Orthidas form an interesting homoe- 

 omorphic pair, but the two can be distinguished readily 

 by their internal features. The ventral musculature of 

 Dolerorthis is hke that of Orthis s. s. and never has the 

 adjuster scars developed to any marked degree. The 

 most important difference, however, is to be seen in 

 the pallial and ovarian impressions of the two genera 

 (see pi. 5, fig. 20, and compare with pi. 10, fig. 24). 

 The pallial marks of Dolerorthis are of the Orthis type, 

 two subparallel trunks given of? from the anterior ends 

 of the diductor tracks and extending directly anteriorly. 

 They are separated by a narrow septal ridge and bound 

 ovarian impressions which occupy nearly the entire sur- 

 face of the interior lateral spaces. In Dinorthis, on 

 the other hand, the ventral pallial trunks are divergent 

 and the ovarian impressions are greatly reduced. 



There is so much variation in the external contour 

 and profile of Dinorthis that several distinct subgenera 

 may be separated as follows: 



1 . D. fectinella — Dinorthis s. s. 



2. D. suhquadrata — Plwsiomys 



3. D. carleyi — Retrorsirostra 



4. D. sola — Pionorthis 



(1) Dinorthis fectinella (pi. 9, figs. 2, 5) is the 

 type of the genus and as well of the subgenus Dinor- 

 this s. s. Its costate exterior differentiates it from 

 the multicostellate exterior of Plwsiomys. 



(2) D. suhquadrata (pi. 9, figs. 3, 20; t. fig. 5) 

 t}'pifies the subgenus that was designated Plirsiomys by 

 Hall and Clarke, and embraces the many species listed 

 above. 



(3) The D. carleyi subgenus (pi. 9, figs. 21-23), 

 now termed Retrorsirostra, is characterized by its 

 strongly prodine ventral palintrope and deeply concave 



ventral valve. In the ventral valve the muscle field 

 is square in front and commonly elevated on a thicken- 

 ing of adventitious shell. The tribe is common in the 

 Upper Ordovician of North America and Europe. 



(4) Another subgenus of Dinorthis is characterized 

 by Orthis sola Billings, referred by Twenhofel"' to 

 Rhipuiontella; it may be called Pionorthis (Gr. fion, 

 fat) in allusion to its biconvex profile (see pi. 9, figs. 

 4, 6-9, t. fig. 18). D. carletotia Twenhofel and an 

 unnamed species from the Upper Ordovician of the 

 Bighorn formation belong to this subgenus. 



The name Dinorthis was used first by Hall and 

 Clarke, who considered its most important characters 

 to be its reversed convexity, subquadrate muscle im- 

 pressions, deltidium, and peculiar cardinal process. 

 The same authors also proposed Plcesiomys, which fol- 

 lows the description of Dinorthis and is characterized 

 by having muscle-scars like those of Dinorthis but an 

 exterior similar to that of Hebertella. It is clear from 



Fig. 18. — Diagram showing the ventral musculature of 

 Dinorthis {Pionorthis) sola (Billings). 



their descriptions that they had O. calligramma in 

 mind when differentiating Dinorthis and were more 

 mindful of the external resemblances of Plirsiomys to 

 Hebertella than of the internal similarities of Dinorthis 

 and PlcFsiomys. 



In the description of the interiors of Dinorthis and 

 Plirsiomys, Hall and Clarke emphasized exactly the 

 same features for both of their genera. This is also 

 borne out by later studies. In an examination of a 

 large series of these shells the present writers have been 

 unable to find any internal generic distinction between 

 the two. Therefore to individualize Dinorthis and 

 Plwsiomys external characters only are available. But 

 here, too, there is considerable diflficulty in the way of 

 a precise definition of the two genera. It was the con- 

 tention of W^inchell and Schuchert that there was a 

 gradation between the external characters of Orthis 

 fectinella (genotype of Dinorthis) and O. suhquad- 

 rata (genotype of Plirsiomys). This contention they 

 embodied in the following words :^*' 



"Mem. 154, Geol. Surv. Canada, 1927 ( 1928), p. 181, 

 pi. 17, figs. 10, 11. 



"Geol. Minn., vol. 3, pt. 1, 1895, p. 421. 



