90 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Description. Exterior. — Shell small or large, 

 subquadrate, semielliptical, anterior margin convex, 

 commonly slightly emarginate, hinge-line wide and 

 straight, cardinal angles straight or obtuse, occasionally 

 subauriculate ; lateral profile unequally biconvex, the 

 dorsal valve being the more convex; anterior com- 

 missure sulcate; ventral fold very low; dorsal sulcus 

 deep; ventral interarea orthocline to catacline, curved 

 or plane, delthyrium open, beak incurved; dorsal inter- 

 area curved, orthocline to apsacline, narrow, beak 

 slightly incurved or straight; ornamentation multi- 

 costate or costate, crossed by strong imbrices which are 

 not uncommonly produced into frills. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity moderately 

 deep, teeth strong, crural fossettes oblique, interarea 

 overhanging the dental plates and not infrequently nar- 

 rowing the delthyrium considerably; dental plates 

 strong in young shells, but obsolete in old ones; muscle 

 area obcordate, strongly impressed, elevated somewhat 

 anteriorly; adductor track broad, central, commonly 

 elevated, nearly equal to or greater than the diductor 

 impressions in length and width; diductor scars nar- 

 row, divergent, elongate; adjustor impressions narrow, 

 elongate, at base of dental plates. PaUial markings 

 orthoid ; two reniform ovarian areas marked by radiat- 

 ing elevated lines occur on each side of the muscle 

 area. A short median elevation extends from the ad- 

 ductors for more than half the distance to the margin, 

 expanding in front. Anterior and lateral margins 

 costellate. 



Dorsal interior. — Notothyrial cavity shallow, cardi- 

 nalia strong; brachiophores orthoid, forming the mar- 

 gins of the notothyrium, supported by adventitious shell 

 substance of the notothyrial platform ; cardinal process 

 a thickened linear ridge, bearing the diductor impres- 

 sions on the lateral surfaces; notothyrial platform 

 thickened, produced forward as a median ridge at 

 least to the middle of the valve; posterior adductor 

 scars smaller than the anterior pair, separated from 

 them by an arched ridge at right angles to ventrally 

 oblique to the median one. Pallial markings consisting 

 of the usual two pairs of curved trunks from the an- 

 terior end of the median ridge, with the addition of two 

 trunks from the outside of the anterior adductors. 



Geologic range. — Early Middle Ordovician 

 (Ridley of Stones River) to Upper Ordovician (Rich- 

 mond). 



American Species 



Dalmanella crispala (Emmons) 1842 



Glyfiorthis insculfta manitoulinensis Foerste 1924 



Hebertella bdlarugosa (Conrad) 1843 



H. Jaylonensis (Foerste) 1885 



H.fausta (Foerste) 1885 



H. insculfta (Hall) 1847 



European Species 



Orthis hdclatchiensis Davidson 1883 

 O. crisfa McCoy 1846 



Discussion. — This group of orthids has ordinarily 

 been identified with Hebertella, but, as pointed out by 

 Foerste in 1914, Orthis insculfta and its allies are 

 totally distinct from that genus and belong in his new 

 genus Glyftorthis. The chief diflFerence pointed out 

 by him between the two genera is the lamellose exterior 

 of Glyptorthis. He also indicated that there was a ten- 

 dency for the dorsal valve of Glyftorthis to be sulcate, 

 whereas the corresponding valve of Hebertella shows a 

 prevalent tendency toward the development of a fold. 

 He concludes: 



In both groups, there are forms in which the median 

 part of the brachial valve is more or less strongly depressed, 

 so as to produce a median sinus, so that the chief distinction 

 of the Hebertella insculfta group is, after all, the presence 

 of the concentric lamellose lines of growth, combined with 

 a shell form which otherwise agrees with that of Hebertella. 



According to our views, Glyftorthis differs from 

 Hebertella in certain features that are far more funda- 

 mental than the imbrication of the growth lamellas. 

 The internal structures at once relate the genus to 

 Orthis and Hesferorthis rather than to Hebertella, and 

 between them the chief differences are as follows: 

 (1) Externally Glyftorthis is always provided with a 

 sulcus in the dorsal valve, a feature not common among 

 orthids. In Hebertella, on the other hand, this fea- 

 ture is characterized by its instability, a fold or sulcus 

 being developed on the dorsal valve in certain species 

 and more uncommonly either one or the other occurs 

 in the same species. 



(2) Internally the ventral valve of Glyftorthis 

 varies importantly from that of Hebertella in its mus- 

 culature and in its pallial markings, and in these fea- 

 tures there is also seen the clue to the taxonomic posi- 

 tion of the former. The ventral musculature is char- 

 acterized by the presence of a full complement of mus- 

 cle-scars, the adjustors being in position at the base of 

 the dental lamellas, but the presence of these muscles 

 in Glyftorthis is a feature not usually seen in the 

 Orthidae. It is probable that in other members of this 

 family the adjustors are joined with the diductors and 

 do not register their presence as a scar on the floor of 

 the valve, or again, they may occupy the sides of the 

 dental plates and therefore have escaped attention. At 

 any rate, they are clearly visible in Glyftorthis and 

 have not been definitely located in Hebertella, in which 

 the diductor scars are expanded in front and the posi- 

 tion of the adductor is represented by a double-tracked 

 ridge upon which are attached the lanceolate adductor 

 scars. This is a considerable variation from the condi- 

 tion seen in Glyftorthis, in which the diductor scars 

 are attenuated in front and rather narrow throughout, 

 and the adductor track is linear, rather flat, and ex- 

 pands anteriorly. In front of the adductor track in 

 Glyfiorthis is a narrow median ridge which expands 

 exactly as in Orthis s. s., Paurorthis, Hesferorthis, 

 Dolerorthisy etc. The prominent kidney-shaped ova- 

 rian impressions of Glyftorthis are striking features of 

 this genus and are never seen in Hebertella. 



