BILLINGSELLIDyE. 739 



Surface marked by distinct plications, with interstitial finer radii, which are crossed by delicate concentric 

 striae; these are usually accompanied by a low sinus and fold on the brachial and pedicle valves, respectively; interior 

 very finely papillose. Shell substance fibrous and apparently punctate." 



The characters of the St. John species are eminently comprehensive; first, the form of the shell is one more fre- 

 quently met with among the strophomenids than among the orthids; the concave plate formed by the union of the 

 dental lamellae is never found in Orthis proper, though occurring in Scenidium. In Orthisina or Clitambonites this 

 plate is always present, but always supported by a median septum and invariably accompanied by the convex del- 

 tidium, which, so far as known, does not exist in Protorthis; while in the group tj^pified by Orthis pepina Hall (here 

 designated by the term Billingsella), the convex deltidium of Clitambonites is present and the concave or dental plate 

 absent. The apparent absence of a cardinal process in Protorthis may be due to the imperfections of the fragile shells 

 studied. The specimens of the St. John shells are preserved as external and internal casts, and from some of these 

 there is reason to infer that the substance of the shell was punctate. 



Type. — Orthis 'hilling si Hartt. 



Ohservations. — ^A study of the various species here described under the genus Protorthis 

 has not given data that will add to the above generic description. All of the species have 

 evidence of punctse of greater or less depth on the inner and outer surfaces of the shell, but in 

 none of them have punctse been found that penetrated through the shell; this may be owing 

 largely to the fact that all of the species are preserved as casts in the sandstone or shale and 

 none of them have the original shell substance or a calcareous or siliceous replacement of it. 



A carefvil examination of a large number of specimens of the dorsal valve fails to reveal, 

 a true cardinal process. 



Protorthis billingsi (Hartt). 



^ Plate XCIX, figures 1, la-g. 



Orthis billingsi Haett, 1868, Acadian Geology, by Dawson, 2d ed., pp. 644-645, fig. 223. (Described. The specimen 

 represented by fig. 223 is not figured in this monograph, but it is redrawn by Walcott, 1884a, PI. I, fig. Ic.) 



Orthis billingsi Hartt, 1878, idem, 3d ed., pp. 644-645, fig. 223. (Copy of preceding reference.) 



OrtteWHng'siHartt, Walcott, 1884, Bull. U.S. Geol. Survey No. 10, pp. 17-18, PI. I, figs. 1, la-d. (Original descrip- 

 tion, Hartt, 1868, pp. 644-645, copied and species discussed. None of the specimens represented by figs. 1, la-d 

 are figured in this monograph. Fig. Ic is drawn from the specimen figured by Hartt, 1868, p. 644, fig. 223.) 



Orthis billingsi.B.&rtt, Matthew, 1886, Trans. Roy. Soc. Canada, 1st ser., vol. 3, sec. 4, No. 4, p. 43. (Discussed.) 



Orthis billingsi Haktt,' 1891, Acadian Geology, by Dawson, 4th ed., pp. 644-645, fig. 223. (Copy of Hartt, 1868, pp. 

 644-645.) 



Orthisina? billingsi Hartt, Matthew, 1891, Trans. Roy. Soc. Canada, Ist ser., vol. 8, sec. 4, No. 6, p. 131. (Discussed 

 and generic reference changed.) ' 



Protorthis billingsi (Hartt), Hall and Clarke, 1892, Eleventh Ann. Rept. State Geologist New York for 1891, p. 274, 

 PI. VIII, figs. 3-7. (Mentioned.) 



Protorthis billingsi (Hartt), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, pp. 219 and 232, 

 PL VIIA, figs. 1^20. (Discussed. Figs. 14^20 are copied from Hall and Clarke, 1892a, PI. VIII, figs. 5, 7, 3, 

 4, and 6, respectively.) 



Billingsella billingsi (Hartt), Schuchert, 1897, Bull. U. S. Geol. Survey No. 87, p. 158. (Merely changes generic 

 reference.) 



Orthis {Protorthis) billingsi (Hartt), Wysogorski, 1900, Zeitschr. Deutsch. geol. GeselL, Bd. 52, p. 227, footnote. 

 (Discussed in German.) 



Protorthis billingsi (Hartt), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 281-282. (Described and discussed as 

 below.) 



Protorthis (Billingsella) billingsi (Hartt), Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 211, 

 fig. 250. (Described. The five figures included in fig. 250 are all copied from Walcott, 1884a, PL I, figs. 1, la-d.) 



The generic description follows very closely that of the type species Protorthis hillingsi in 

 its main features. All of the specimens are compressed in the embedding shale, which renders 

 it difficult to get a true conception of the convexity and form of the valves. The general form 

 is shown by the illustrations. The outlme may be transversely quadrilateral or subsemicircidar 

 (PL XCIX, figs. 1 and le) or subquadrate (figs. la-f). Usually the beak is inclmed backward 

 over the area, but it may be distorted by pressure so as to appear to incline forward. Young 

 shells have a weU-defiiied median sinus on the ventral valve that shows as a flattened space on 

 the larger shells. A slight sinus sometimes appears on the dorsal valve. The surface of the 

 shell varies in the number and size of the radiating ribs; sometimes they are scarcely visible 



a The casts of the interior of the valves indicate that the inner layer of the shell is punctate, but I have been unable to obtain any evidence 

 that the shell had the fibrous or punctate structure characteristic of Dalmanella. 



