ACEOTRETID^. 651 



Linnarssonia? rdtida (Ford), Walcott, 1901, Proc. U. S. Nat. Mus., vol. 23, p. 673. (Merely changes genericreference.) 

 Obolella nitida Ford, Grabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 189, fig. 223c. (Described. 

 Fig. 223c is copied from Walcott, 1886, PI. XI, fig. 2.) 



The original description by Ford follows: 



Shell transversely suboval, small. Dorsal valve gently but irregularly convex, the greatest elevation occurring 

 at a point about one-fifth the length of the valve from the apex. From this point the beak curves sharply down to 

 the hinge line, which it almost touches. The hinge line itself is slightly curved and apparently equal to about one- 

 third the width of the shell. At the most elevated point of the valve commences a well-defined median depression, 

 which extends forward for a distance of about one-half the length of the valve, gradually widening and becoming 

 more shallow until it disappears. A portion of the dorsal valve close to the margin is sometimes nearly flat all around. 

 The internal markings are not well enough shown in any of the specimens that I have seen to admit of description. 

 The surface is ornamented with very fine concentric striae and niimerous close set radiating strise, the whole just visible 

 to the unassisted eye. 



The ventral valve is not certainly known. The width of the largest dorsal valve that I have seen is 0.14 of an 

 inch and the length 0.1 of an inch. 



Through the courtesy of Prof. J. M. Clarke I have had tlie opportunity of studying the 

 type material of this species, now in the New York State Museum collections. There are 

 four specimens of the dorsal valve and one representing the ventral valve. The reference 

 to Obolella [Ford, 1873, p. 213] and Linnarssonia [Walcott, 1901, p. 673] is incorrect. The 

 shell is like that of Acrothele and the matrix representing the ventral valve shows all the 

 characters of Acrothele as far as any characters are shown at all. The slope of the valve toward 

 an eccentric apex is indicated in the same manner as in Acrothele matthewi (Hartt). 



The only difference detected between this shell and young sheUs of Botsfordia cselata 

 (Hall) is the absence of the characteristic pustulose surface of the latter. Five of Mr. Ford's 

 specimens appear to have come from the same piece of limestone, and nothing that can be 

 identified as the same species has been found in the large collection made at the same locahty. 



The largest shell has a diameter of 3 mm. The surface is marked by fine concentric strise, 

 and sharp, interrupted, radiating hnes. 



Formation and locality. — Lower Camhrian : (27) Even-bedded and conglomerate limestones on the ridge in the 

 eastern suburb of Vroij, Rensselaer County; and (22) limestone 1.5 miles (2.4 km.) north of North Granville, Fort Ann 

 quadrangle (U. S. Geol. Survey), Washington County; both in New York. 



Acrothele panderi n. sp. 

 Plate LIX, figures 5, 5a-c. 



Tliis species is of the same general type as Acrothele colleni n. sp. It differs in having 

 a less elevated and more posterior apex on the ventral valve, and the finer granulation of the 

 exterior of the shell; also in the presence of a median depression or sinus on the dorsal valve. 



The surface is more like that of Acrothele (Redlichella) granulata (Linnarsson) as the granu- 

 lation, although of the same type as that on A. coriacea Linnarsson and A. colleni, has more 

 the appearance of an epidermal granulated layer. 



The description of A. colleni applies to tliis shell with the exceptions mentioned, and a 

 few other details. It also reaches a somewhat larger size. One specimen of the ventral valve 

 has a transverse diameter of 11 mm., with a length of 9 mm. Another example of the same 

 valve has an equal length and breadth; usually the dorsal valve is a little broader than long, 

 and it is marked by a clearly defined mecUan depression that extends from the beak forward, 

 gradually widening to the front margin. The apex of the ventral valve has a minute tubercle 

 on each side, with a small pedicle opening on the slope just back of the tubercles, in this respect 

 closely resembling the apex of A. coriacea. 



The interior of the ventral valve shows a tliickening of the shell under the short visceral 

 area in front of the pedicle opening; the main vascular sinuses appear to originate beside or a 

 Uttle in advance of the pedicle opening and extend forward, gradually separating, but occupy- 

 ing a position about halfway between the margins and the median line. 



