A NEW SPECIES OF OLENELLUS 269 



XXXI, fig. I, the margins of the incision are pushed together and the 

 pygidium narrowed. In the one case, pressure on the matrix was 

 exerted longitudinally, and in the other, laterally. Thus in the two 

 specimens are presented opposite effects of like pressure, results that 

 must be considered in forming a conception of the shape of a specimen 

 free from all distortion due to pressure. 



For about half the distance to the posterior end, the pygidium 

 presents a slightly raised surface. The curved lines, on each side of 

 the thoracic segment, marking the ends of the pleural grooves, extended 

 to meet, follow the margin of this overlapping fold and unite at its 

 middle point. 



The anterior half of the pygidium is marked by transverse lines 

 similar to those on the falcate ends of the pleura. 



Associated with the entire specimen (PL xxxi, fig. i) is a doublure 

 with hypostoma attached. The doublure (PI. xxxii, fig. 3) is 

 partly overlapped by the cephalic shield of which it was evidently 

 once a part. Its inner margin is beaded by a row of closely placed 

 and deep indentations which are still more strikingly and clearly pre- 

 sented in part of another doublure (PI. xxxii, fig. i). 



The attached hypostoma, whilst distorted, presents enough of its 

 serrated lateral margin to identify it with the entire specimen (PL 



XXXII, fig. 3) from the same locality. 



The immediate anterior margin is entire, but its lateral extension, on 

 each side, is serrated, bearing six teeth. The number of teeth, as de- 

 termined by all the hypostomas I have thus far found, seems to be 

 constant. Moreover the fact that, in the same localities, specimens of 

 Olenellus thompsoni are found in greater abundance than those of 

 Olenellus ivalcottanus^ and that the type (PL xxxii, fig. 3) illus- 

 trates the only form of hypostoma observed, justifies the conclusion 

 that the hypostomas of both species are essentially the same in charac- 

 teristic features. 



Formation and Localities. — Lower Cambrian, argillaceous shales, 

 about three miles north and northwest of York, Pennsylvania, near the 

 contact line of Cambrian and Trias. 



