OLENELLUS AND OTHER GENERA OF MESONACIDJE 303 



brian terrane. It is quite abundantly represented by fragments in tbe 

 collections from York and Lancaster counties, Pennsylvania, and more 

 rarely by entire specimens. Since the original description of the 

 species [Wanner. 1901] Professor Wanner has found specimens that 

 prove the existence of a median spine on the fifteenth segment that 

 in old and large dorsal shields is as strong as in large dorsal shields 

 of P<Tdcu})iias traiisifaus [compare fig. 11, pi. 30, with figs. 3 and 4, 

 p'- 33]- O" smaller dorsal shields the median spine is proportionallv 

 much less developed [see pi. 30, figs. 10 and 12]. The median spine 

 of the fourteenth segment is short and slender, but stronger in large 

 dorsal shields than the pointed nodes on the other segments of the 

 thorax and the occipital segment of the cephalon. 



The palpebral lobe of the adult is small, about one-third of the 

 length of the cephalon [fig. 2], but these specimens of vounger 

 stages of growth indicate that the lobe is progressively longer [figs. 

 3, 4] as the cephalon decreases in size. This character is finely 

 shown in the young of IVaiiiieria Jialli [pi. 31]. 



The presence of the great spine on the fifteenth segment indicates 

 the approach of the Mesonacis stage of development and the tendencv 

 to acquire the adult character of Olenellus thompsoni of having a 

 large terminal telson without segments and pygidium posterior to it. 

 The adult IV. wakottanns resembles Callavia brdggeri (Walcott) 

 and C. callavci (Lapworth), but dififers greatly in its smaller eyes, 

 absence of occipital spine, and presence of a great spine on the 

 fifteenth segment. 



DiniciisiGiis. — A large somewhat flattened dorsal shield from i 

 mile north of Rohrerstown, Lancaster County, Pennsylvania, has a 

 length of 17.6 cm. and a width at the genal angles of the cephalon 

 of 15 cm. The cephalon is 6.4 cm.; thorax 10.2 cm., and pvgidium 

 I cm. in length. The e}e lobes vary slightly in length as compared 

 with the length of the cephalon, but the average length is one-third 

 of the length of the cephalon. The relative proportions of other 

 parts of the dorsal shields are well shown b}- fig. i, pi. 30. 



The cephalons of Olenellus thompsoni cmssiinargiuatiis [pi. 35, 

 figs. 8-10] recall those of IV. ivalcottanus [pi. 30], except that the 

 latter has small eyes and an expanded anterior glabellar lobe, while 

 the former has large eyes and a narrower anterior lobe to the glabella. 



This species differs from Wanncria halli [pi. 31] in having a wider 

 anterior glabellar lobe, proportionally wider glabella, narrower 

 cheeks, with the genal angles on a line with the posterior margin 

 of the cephalon. 



