GENERA OF THE SUBORDER ORTHOIDEA 



91 



Generic and family differences distinguishing He- 

 brrtfUa from Glyptorlhis are to be found in the dorsal 

 valves. The brachiophores, and especially the cardinal 

 process of the last named genus, arc of the orthoid 

 type. In Hfherteliti, on the other hand, the cardinalia 

 are composed of brachiophores intimately combined 

 with distinct convergent supporting plates which unite 

 with the floor of the valve at the base of the cardinal 

 process, while in Glyptorthis the orthoid brachiophores 

 are supported by adventitious shell deposited in the 

 notothyrial cavity, as in all other Orthid.-e. Rather 

 deep adductor pits and the scars of attachment of one 

 set of the diductors at the base of the cardinal process 

 have so excavated the notothyrial platform as to make 

 it simulate the condition seen in Hebertella. 



The pallial markings in the dorsal valve of Glyptor- 

 this vary from the simple markings of Orthis s. s. and 

 Cyrtonotella. In the former the four trunks are given 

 off from the anterior ends of the diductors but are 

 bifurcated at their inception. Subsidiary rami are 

 developed along the front margins. 



Glyfiorth'u first appears in the early Middle Ordo- 

 vician (Ridley member of the Stones River) in Ken- 

 tucky and Tennessee, and ranges into the Richmond 

 division of the Upper Ordovician. In the Girvan dis- 

 trict of Scotland the genus appears first in the Stinchar 

 limestone (=Chazy) and ranges into the Drummock 

 group (^Richmond). 



Genus ERIDORTHIS Foerste 1909 



PI. 5, figs. 1,2,5,9 



Foerste, Bull. Sci. Lab. Denison Univ., vol. 14, 1909, p. 

 223, pi. 4, figs. 3a-d. 



Genolectotype (Schuchert and LeVene 1929). 

 — Plectorthis (Eridorthis) nicklesi Foerste 1909. 



Description. Exterior. — Very similar in general 

 physiognomy to Glyptorthis, but varying in important 

 details. Outline semicircular, cardinal extremities ob- 

 tuse or rounded, slightly deflected; lateral profile 

 commonly subequally biconvex, with the ventral valve 

 usually slightly more convex; in young shells there is 

 a median fold on the ventral valve and a sulcus on the 

 dorsal, but in mature individuals the latter becomes a 

 more or less prominent fold, while the fold of the 

 opposite valve is anteriorly depressed. Ventral inter- 

 area the higher, curved and strongly apsacline, umbo 

 convex. Dorsal interarea short, apsacline, notothyrium 

 open. Ornamentation multicostate, with prominent 

 concentric lamellae as in Glyptorthis. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity shallow; teeth 

 strong; dental plates well defined in mature shells; 

 muscle area placed on a low callosity, cordate or sub- 

 triangular, adductor track linear, expanding anteriorly. 

 Diductor tracks elongate, narrow; adjuster scars nar- 

 row when visible. Pallial and ovarian impressions 

 obscure, but when visible, much as in Glyptorthis. 



Dorsal interior. — Cardinalia very confined; brach- 

 iophores of the Orthis type, short, blunt, supported by 

 the notothyrial platform; cardinal process a simple 

 linear ridge; median ridge extending about one-half 

 the length of the valve ; muscle area small, quadripar- 

 tite; pallial and ovarian impressions obsolete. 



Geologic range. — Middle Ordovician and early 

 Silurian of North America. 



Species 



Hebertella {Eridorthis) nicklesi (Foerste) 1909 

 H. (E.) rogersensis (Foerste) 1909 



Discussion. — Foerste separated this assemblage as 

 a subgeneric group under Plectorthis chiefly on the 

 basis of external features. The internal characters, 

 however, show it to have no relationship with Plector- 

 this, but to be related genetically to Glyptorthis and 

 the Orthida; in general. Eridorthis differs from the 

 other members of the Glyptorthina chiefly in its ex- 

 ternal characteristics. The dorsal valves of Glyptor- 

 this and Ptychopleurella are always marked by a sulcus, 

 but in Eridorthis the dorsal sulcus reverts into a fold 

 or is lost in the general convexity of the valve. Exter- 

 nally it differs from Plectorthis in the characters 

 of the fold and sulcus and likewise in the rugose 

 ornamentation. 



The interior of the ventral valve has Orthis char- 

 acters as seen in the low median ridge, traces of reni- 

 form ovarian impressions, and subparallel pallial im- 

 pressions. The brachiophores and cardinal process are 

 distinctly of the Orthis type. These are the most im- 

 portant differences between Eridorthis and Plectorthis 

 and absolutely preclude the placing of the former as 

 a subgenus of the latter and set it apart in a separate 

 subfamily. 



From Glyptorthis this genus differs mainly in the 

 reversion of the fold and sulcus and in the lesser devel- 

 opment of the ovarian and pallial impressions. E. nick- 

 lesi and E. rogersensis were first referred by Foerste to 

 Plectorthis, and later all four of the species were listed 

 by Bassler under Hebertella. It must be emphasized, 

 however, that Eridorthis has none of the typical fea- 

 tures of Hebertella as exhibited in the genotype of that 

 genus. The ventral musculature and what traces 

 of the pallial and ovarian markings are present clearly 

 indicate affinities with Hesperorthis, Dolerorthis, Schiz- 

 oramma, and other genera of the Orthidas. 



Externally, the degree to which the development of 

 fold and sulcus is carried is an important distinguishing 

 characteristic of Eridorthis. In the dorsal valve of 

 young shells there is a shallow sulcus bordered by the 

 first radiating costs. This sulcus may extend for 

 3-5 mm., and then with the implantation of the first 

 ribs on the inside of the primary marginal costae, the 

 sulcus becomes an elevated median fascicle. On the 

 ventral valve the median costa is elevated at the beak, 



