464 CAMBKIAN BRACHIOPODA. 



Obolus (Lingulella) rogersi Walcott, 1898, Proc. U. S. Nat. Mu8., vol. 21, pp. 413-415. (Described and discussed 



essentially as below as a new species.) 

 Lingulella rogersi (Walcott), Geabau, 1900, Occas. Papers Boston Soc. Nat. Hist., No. 4, vol. 1, pt. 3, pp. 624-625, PI. 



XXXI. fig. 4. (Described and discussed.) 

 Obolus (Weslonia) rogersi Waloott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 691. (Merely changes generic reference.) 



General form elongate ovate, with the ventral valve subacuminate and the dorsal valve 

 ovate in outliae. There is considerable range of variation in the outline of the valves, owing 

 largely to distortion apparently produced by movement of the matrix. The convexity of the 

 valves is fairly strong and nearly the same in both, except that the dorsal valve curves more 

 abruptly inward toward the beak. 



The outer surface of the shell usually adheres to the matrix, but, in three specimens, portions 

 of it are preserved which show that it is of essentially the same character as that of Oholus 

 ( Westonia) stoneanus (PI. XXVIII). The surface is formed by very fine concentric lines and 

 strife of growth crossed transversely by strong, undulating, slightly lamellose lines (PI. XLII, 

 figs. 2 and 2d). When the outer layer is exfoliated the inner layers are marked by concentric 

 lines of growth and fine radiating striae. This is also the character of the inner surface, so far as 

 can be determined from the specimens in the collection. The shell is rather thick and btiilt 

 up of a thin outer layer and several inner layers or lamellse, the latter becoming increasingly 

 nmnerous toward the fi'ont. The largest dorsal valve in the collection has a length of 13 mm. 

 with a width of 11 mm., and a smaller ventral valve with a length of 12 mm. has a width of 

 9 mm. The dimensions of most of the specimens in the collection average less than those 

 here given. 



The area of the ventral valve is relatively short for a species of this type. It is divided 

 midway, as seen in the cast, by a strong pedicle furrow. Owing to the imperfection of the mate- 

 rial, none of the specimens show flexure lines or strise of growth. The area of the dorsal valve is 

 short and extends but a short distance on either side of the median line. The cast of the interior 

 of the ventral valve is very much like that of the interior of Obolus cyane (PI. XVII). It has the 

 same median ridge and the transverse trapezoidal area which includes the central, middle lateral, 

 and outside lateral muscle scars. The main vascular smuses are indicated by slight ridges 

 (PI. XLII, figs. 2a and 2b). The cast of the interior of the dorsal valve shows a narrow median 

 septum, two central muscle scars of average size, situated a short distance back of the center of 

 the shell, and two small anterior lateral scars, located some distance in advance of the center, 

 wliich give an elongated visceral cavity somewhat like that of Lingulella hayesi (PL XXV) of 

 the Middle Cambrian and Obolus lamborni (PI. XXII) of the Upper Cambrian. 



Observations. — The external form of the more elongate specimens of this species is very 

 much like that of Lingulella acutangula (Roemer) (PI. XVII). When compressed laterally it 

 occasionally has the form of Lingulella (Lingulepis) acuminata (Conrad). It is distinguished, 

 however, from all described species of Westonia known to me bj^ its highly characteristic 

 surface ornamentation. Obolus (Westonia) stoneanus (PI. XXVIII) has the same type of 

 surface, but it differs from the latter in being a much more elongate shell. 



The material described in this monograph was collected by Prof. N. S. Shaler and Mr. J. B. 

 Woodworth from the pebbles on the beach on the northern shore of Marthas Vineyard, Massa- 

 chusetts, and at several points along the shores of Narragansett Bay. 



The first notice we liave of these fossiliferous pebbles is that by W. B. Rogers, who [1861, 

 p. 389] announced the discovery, by Norman Easton, of pebbles carrying fossils of the Potsdam 

 fauna in the Carboniferous conglomerate north of Fall River, Massachusetts. In 1875 [p. 100] 

 Rogers announced the discovery of impressions, suggestive of the fossil Lingulas mentioned by 

 him from Fall River, in the pebbles in the conglomerate at Newport, Rhode Island. He 

 thought that the pebbles were derived from rocks probably closely connected in time with 

 the Brain tree Paradoxides zone. 



Among the material sent by Professor Shaler I fomid the remains of a large Linguloid 

 brachiopod, which appears to be identical with Obolus (Lingulobolus) affinis (Billings), from 

 the Lower Ordovician rocks of Newfoundland. The material is somewhat imperfect, but I 



