108 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Genus DELTATRETA Ulrich 1926 



PI. 6, figs. 10, 11, 13-15, 19, 30 



Ulrich in Butts, Geol. Surv. Alabama, Spec. Rept. No. 14, 

 1926, p. 100, pi. 18, figs. 1-+ (no description and 

 no type selected). 



Syn. Deltorthis Ulrich, in Poulsen, Meddel. om Gronland, 

 Bd. 70, 1927, p. 297, pi. 20, fig. 4. 



Genolectotype. — Deltatreta fillistriata Butts 

 1926. 



Description. Exterior. — Resembling Pionodema 

 externally, subelliptical, hinge-line straight, usually nar- 

 rower than the greatest shell width; cardinal extremi- 

 ties obtusely subangular. Lateral profile unequally 

 biconvex, the ventral valve usually having the greater 

 convexity. Anterior commissure rectimarginate. Ven- 

 tral palintrope long, curved, moderately or strongly 

 apsacline; delthyrium closed by a strongly convex 

 deltidium which is perforated near the apex by a large 

 foramen like that of Vellamo and the Clitambonitidas. 

 Dorsal interarea short, anacline ; notothyrium partially 

 closed by discrete chilidial plates. Surface multicostel- 

 late; shell structure fibrous, impunctate. 



Ventral interior. — Delthyrial cavity deep ; teeth 

 strong, crural fossettes prominent ; dental plates strong, 

 extending directly to the floor of the valve, defined by 

 deep umbonal cavities. Muscle marks strongly im- 

 pressed on a callus which grows under them and 

 spreads about the inner bases of the dental plates, form- 

 ing an incomplete pseudospondylium. Diductor scars 

 elongate, situated near the base of the dental plates, 

 not extended in front of the adductor track, the latter 

 occupying the space between the adductor impressions 

 and in mature and old shells continued forward as a 

 low ridge. Lateral spaces marked by subreniform 

 ovarian impressions as in Orthis s. s. and Hemipronites. 



Dorsal interior. — Cardinalia confined to the vicinity 

 of the hinge. Cardinal process orthoid, a linear 

 septum. Brachiophores orthoid, supported by lateral 

 swellings of adventitious shell exactly as in Vellamo 

 and Clitambonites. 



Geologic range. — Lower Ordovician (Canadian) 

 of the United States and Canada. 



Species 



D. fillistriata Butts 1926 

 D. two n. spp. Ulrich MS. 

 Billingsella dice Walcott 1 90 5 



Discussion. — The combination of characters in 

 this interesting early Ordovician shell unites features 

 seen in the later Orthidx and the Clitambonitidae. 

 Orthoid features are the ovarian markings and discrete 

 dental plates, clitambonitoid ones are the apically per- 

 forate deltidium and the brachiophores supported by 

 lateral extensions of adventitious shell as in Vellamo. 



We will first describe in detail the morphology of 

 this genus and then discuss the peculiar nomenclatorial 

 questions involved in the choice of a genotype. Ex- 

 ternally Deltatreta has the appearance of Pionodema 

 in both outline and profile and the shell is finely multi- 

 costeUate with the ventral palintrope curved and apsa- 

 cline as in the last named genus. The ventral inter- 

 area is marked by fine, elevated lines parallel to the 

 posterior margins of the shell. Such elevated lines are 

 also characteristic of Polytcechia. Deltatreta is unlike 

 the later Pionodema in its possession of a perforate del- 

 tidium. Such a deltidium is unknown so far in any of 

 the punctate shells such as Pionodema. The deltidium 

 of Deltatreta is strongly convex, decidedly like the 

 same feature in Clitambonites, and unlike that of Bill- 

 ingsella since the apical foramen is very large, and not 

 uncommonly the foraminal margins are flexed or 

 turned outward. 



In the ventral valve the musculature and pseudo- 

 spondylium are variable, depending on the age of the 

 specimen. In young shells there is no vestige of a 

 pseudospondylium, the adductor and diductor tracks 

 are rather wide, and there is a prominent pedicle callist 

 showing the inner attachment of the pedicle (see pi. 6, 

 fig. 30). In old shells the space occupied by the 

 adductor field is strongly elevated on adventitious shell, 

 which is not uncommonly prolonged forward to the 

 middle of the valve as a thick median ridge (pi. 6, 

 figs. 13, 15). The latter may nearly completely cover 

 the floor of the delthyrial cavity, forming a rather 

 prominent pseudospondylium. The floor of the lateral 

 spaces and umbonal cavities is marked by low radiating 

 ridges, probably markings of the attachments of the 

 ovarian bodies. In one specimen (see pi. 6, fig. 15) 

 the ovarian impressions are subreniform as in Orthis 

 s. s. or Hesferorthis. The palintrope and dental plates 

 are not unlike the corresponding structures in Hes-per- 

 orthis. The palintrope is long, and the teeth are placed 

 considerably outside of the delthyrial margins. They 

 are rather small but have prominent crural fossettes. 

 The deltidium develops as in Hesperorthis and Clitam- 

 bonites. It is an arch built with its sides attached to 

 the ventral surface of the narrow flange of the palin- 

 trope which overhangs the delthyrial margins. The 

 scar of pedicle attachment is not confined to the callist 

 at the apex but can be traced on the sides of the dental 

 plates and on the ventral surface of the deltidium; on 

 the dental plates its margin runs from the antero- 

 lateral extremity of the callist as a low ridge in an 

 antero-dorsal direction, nearly parallel with the sloping 

 edge of the dental plates, and then passes to the ventral 

 surface of the deltidium some distance behind the 

 foramen. 



The dorsal valve is usually less convex than the 

 ventral and has a shorter palintrope, and the notothy- 

 rial cavity is rather narrow and shallow. The noto- 

 thyrial platform is inconspicuous and bears a simple 

 cardinal process which is typically orthoid. The brach- 



