GENERA OF THE SUBORDER ORTHOIDEA 



89 



Dorsal interior. — Notothyrial cavity shallow, cardi- 

 nalia strong; brachiophorcs strong, broadly rounded in 

 front, of Orthis type, and bearing flat, bluntly pointed 

 plates on the outside distal extremity. Cardinal process 

 a simple linear ridge; a median ridge extends from 

 the notothyrial platform to the middle of the shell. 

 Muscle area small; anterior adductors smaller th.in 

 the posterior ones, irregular; posterior adductor track 

 broad and long. Ovarian impressions occupy the 

 internal lateral spaces as reniform areas. 



Geologic range. — Lower and Middle Silurian. 



American Species 



Orthis flabellites Foerste 1889 

 O.fiabelliies dinorthis (Foerste) 1895 

 O.fiabellites euorlhis (Foerste) 1895 

 O. jUbellites fissiplicata (Foerste) 1 895 

 O. fiabellites militaris Foerste 1909 

 O. inter flicata Foerste 1909 

 O. rtetielrothi Foerste 1909 



European Species 



Orthis rustic a Sowerby 1839 

 O.rustica osUiensis Schrenk 1858 



Distinguishing characters. — The characteris- 

 tics used in separating Dolerorthis from other orthid 

 genera are: (1) The convexity of the valves is the 

 reverse of that normally seen in Orthis s. s. or Hesfer- 

 orth'u; (2) the dorsal valve is similar to that of 

 Dinorthis in all but the cardinal process. Family rela- 

 tionships with the Orthida: are seen in the ventral 

 muscle field, which has a pattern like that of Orthis 

 s. s., and essentially as in Hesperorthis. Furthermore, 

 Dolerorthis has in the ventral valve the low, narrow, 

 forked median septum, the pallial trunks and reniform 

 ovarian impressions of Orthis. In the dorsal valve, 

 affinities with the Orthidae are seen in the brachiophorcs. 



Dolerorthis is readily distinguished from its sub- 

 family companions, Hesperorthis and Schizoramma, in 

 having the relative convexity of the valves reversed, 

 but is not resupinate as are the strophomenids. The 

 disproportionate convexity between the two valves, the 

 dorsal being the more convex, is a consequence of this 

 reversion, and brings about the superficial external 

 resemblance to Dinorthis. There is, however, a rather 

 marked difference in the cardinal process of Dinorthis 

 and Dolerorthis, since in the latter genus it is a simple 

 ridge, while in the former it is thicker and the posterior 

 surface is crenulated, showing that the muscle attach- 

 ments were on this portion of the process and not on 

 its sides or at its base. 



Discussion. — Dolerorthis is another group of brach- 

 iopods showing effectively the disconcerting role played 

 by homceomorphy. Within the group are found hom- 

 oeomorphs of Plcesiomys and Dinorthis. For many 

 years Orthis flabellites was mistakenly identified with 



O. flnhrllulian (Sowerby) but in 1867 Hall" changed 

 the name to fliibtltite-s, and Foerste^** did the same in 

 1889 when he pointed out the anatomical differences. 

 Hall and Clarke^" regarded O. flabellites as "a form 

 connecting the typical Orthides with Dinorthis; 

 though, in geological time, appearing at the end of the 

 two groups." Had they realized the presence of 

 homa'omorphy they would have recognized at once 

 the uniqueness of Dolerorthis. The latter differs from 

 Ditwrthis in both ventral and dorsal valves, in features 

 which are of family import. In the first place, Doler- 

 orthis does not have the quadrate muscle field of the 

 other group, and besides, in Dinorthis there are two 

 pallial trunks extending antero-laterally from the di- 

 ductor scars, which in their passage toward the front 

 break up into numerous subsidiary rami, and there are 

 no kidney-shaped ovarian marks occupying the umbo- 

 lateral spaces. Again, Dolerorthis does not have the 

 crenulated cardinal process. On the other hand, the 

 multicostate species D. interplicata and D. rustica have 

 a superficial resemblance to some members of Plte- 

 siomys, but the homoeomorphy in these instances is 

 less striking than that shown by Orthis flabellites and 

 Dinorthis flabellulum. 



It may be well to say here that some of the shells 

 now passing under the name Orthis flabellites do not 

 conform with the shells that occur in New York and 

 Indiana in the Middle Silurian. These are of the 

 older Cataract of Ontario, and they are smaller than 

 the type species and are quite clearly biconvex. Like 

 many other "comfortable" species, this one is in sore 

 need of revision in the light of modern paleontology. 



Subfamily GLYPTORTHIN.€ Schuchert and 

 Cooper 1931 



Specialized Orthidae with the general internal ex- 

 pression of the Hesperorthina but with a rugose ex- 

 ternal surface. Deltidium and chilidium absent. This 

 small family is composed of three closely related 

 genera. Eridorthis appears to be a Glyptorthis which 

 has accomplished a reversion of the fold and sulcus 

 toward the front of the shell. Ptychopleurella is an 

 off-shoot from Glyptorthis, with an unusual subspiri- 

 feroid contour and profile. 



Genus GLYPTORTHIS Foerste 1914 



PI. 5, figs. 4, 7, 8; pi. 6, figs. 12, 17, 18, 20, 21, 

 26,29 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 17, 1914, p. 258. 



Genoholotype. — Orthis insculpta Hall 1847, Pal. 

 N. Y., vol. l,p. 125, pi. 32, fig. 12. 



" 20th Ann. Rcpt. N. Y. State Cab., p. 436; see also 

 Palcontologv of New York, vol. 8, pt. I, p. 227. 

 ** Proc. Boston Soc. Nat. Hist., vol. 24, p. 31 1. 

 "Pal. N. Y., vol. 8, pt. 1, p. 227. 



