GENERA OF THE SUBORDER ORTHOIDEA 



63 



Genus DOLEROIDES Cooper 1930 



PI. 11, figs. 6-8, 10-13, 16, 18 



Cooper, Jour. Pal., vol. 4, 1930, pp. 375, 382, pi. 35, figs. 

 5-7, 14; pi. 37, fig. 2. Also D. fert'etus, p. 381, 

 pi. 35, fig. 8; pi. 36, figs. 4, 7; pi. 37, figs. 1, 3, 13. 



Genoholotype. — Orthis gibbosa Bi!linp;s 1857, 

 Geol. Surv. Canada, Rept. Progress for 1856, p. 296. 



Description. Exterior. — Externally Doleroides 

 corresponds closely to Pionodema, being lenticular or 

 nearly so in profile and subsemielliptical in outline. 

 The ventral interarea is the longer, the delthyrium is 

 open, and the notothyrium is modified only by the 

 cardinal process. The surface of the shell is multi- 

 costellate, with numerous hollow costcllas; test fibrous, 

 impunctate. 



Ventral interior. — Dental lamells prominent; di- 

 ductor impressions subcrescentic, elongate, surrounded 

 on the outside by a thickened margin ; adductor ridge 

 single; adductor field elliptical; adjustor impressions 

 elongate, narrow. Umbo-lateral spaces marked by 

 radial ovarian ridges. The entire inner surface of the 

 shell is marked by low radial ridges and elongate 

 pustules. 



Dorsal interior. — The cardinal process is a thick 

 ridge with a plectorthoid myophore, long blunt brach- 

 iophores with convergent supporting plates forming a 

 shallow sessile cruralium. Concave fulcral plates unite 

 the brachiophores to the walls of the valve and define 

 the sockets. Umbo-lateral spaces marked by inter- 

 rupted, wavy, radial ridges. 



Geologic range. — Lower Middle Ordovician 

 (Black River) of North America. 



Species 



Ddmanella subttquata gibbosa (Billings) 1857 

 D. subaquala ferveta (Conrad) 1843 

 Undescribed species 



Discussion. — It is evident from the internal fea- 

 tures of this genus that it is very closely related to 

 Hebertella, differing chiefly in its subequal biconvexity 

 and ventral musculature. The latter differs from that 

 of Hebertella chiefly in lacking the double median ad- 

 ductor ridge and in having the adjustor impressions 

 more strongly developed. The dorsal interior is identi- 

 cal with that of Hebertella. 



This small group of plectorthids forms a remarkable 

 example of homceomorphy. So precise is the mimicry 

 of Pionodema by D. gibbosa that this species has usually 

 been identified as "Pionodema subwquata gibbosa 

 (Billings)." The occurrence of these shells side by 

 side (isochronous homoeomorphy) further militates 

 against their accurate identification. The mimicry is 



carried so far that Doleroides even has an abundance 

 of hollow costelLx, although they are not quite so 

 numerous as in Pionodema. 



The chief external distinguishing features between 

 Doleroides and Pionodema are the almost uniformly 

 wider hinge-line, the more pronounced and more uni- 

 versal fold and sulcus, and the less abundant hollow 

 costelhe in the former. Further, the costclla: in Doler- 

 oides are in general coarser than those of Pionodema in 

 shells of about the same size. Internally, however, 

 the separation of the two is easy. In the ventral valve 

 of Pionodema there is a small apical plate, the diductor 

 impressions are subflabellate, and the adductor ridge is 

 extended in front of the anterior margin of the muscle 

 field. In Doleroides, on the other hand, there is no 

 apical plate, the muscle field is bounded by an elevated 

 border, and the adductor ridge is confined to the 

 muscle field (compare fig. 6, pi. 1 1 with fig. 9, pi. 

 23). In the dorsal valve the brachiophore supports 

 of Pionodema are divergent, not convergent as in 

 Doleroides (compare fig. 12, pi. 11 with figs. 7, 8, 

 pi. 23). The final and most conclusive test of generic 

 dissimilarity between these homoeomorphs is, of course, 

 a thin section of the shell, Pionodema being punctate. 



In general appearance, Doleroides is similar to 

 Schizofhorella of Europe, but there are important dif- 

 ferences. Externally the American shells do not have 

 the strong development of the fold and sulcus char- 

 acteristic of the European species. D. gibbosus shows 

 a rather prominent fold and sulcus but in D. pervetus, 

 as understood, the dorsal valve is provided with a sulcus 

 in young stages which is lost later on at the front of 

 the valve. In some specimens, however, there is a 

 distinct dorsal fold in the mature shells, and in rare 

 instances a sulcus may be present on both valves, pro- 

 ducing an emargination of the anterior margin. They 

 thus show the same instability of fold and sulcus ex- 

 hibited by Hebertella and other more primitive genera 

 of brachiopods. 



Internally the variation of Schizophorella from 

 Doleroides is more pronounced. In the ventral valve 

 of some specimens the adductor ridge is continued for- 

 ward beyond the confines of the adductor field nearly 

 to the front margin of the shell, and in the dorsal 

 valve the brachiophore supports are always convergent 

 but may remain discrete at their union with the floor 

 of the valve. This condition may also be observed in 

 the young of Plectorthis, in which these plates either 

 unite with the floor and remain slightly separated, or 

 unite with each other and with the floor of the valve 

 simultaneously. In Schizophorella both conditions 

 occur in the same species. If we add to these differ- 

 ences a later geological age and a different geographical 

 province, we see at once the full significance of these 

 small variations. Doleroides is chiefly at home in Mid- 

 dle Ordovician (Black River) rocks of North America, 

 whereas Schizophorella occurs in the Upper Ordovician 

 of the British Isles. 



