42 BRITISH CAMBRIAN TRILOBITES. 



" Many specimens are found inrolled. In this case the spines on the fourth 

 segment lie back in the plane of the tail, the last three segments thus taking little 

 part in the rolling. In agreement with this it can be seen that the skin is more 

 deeply infolded at the joints in front of the fourth segment than behind. The 

 folds in front of the fourth segment are in consequence often broken away. 



"Dimensions of a perfect specimen collected by Mr. Rhodes. Length : total, 

 2*9 mm.; head, 1*03 mm.; thorax, 1*2 mm.; tail, OG7 mm. Breadth: head, 

 2 - 2 mm.; tail, 1'5 mm. 



"Development. — Among the material from the Shineton Shales, collected by Mr. 

 Rhodes for the Geological Survey, are two individuals, one 1 mm. (R. R. 2286) and 

 the other 1*5 mm. (R. R. 2296) long. In these the head is semicircular in outline, 

 with the same features as the above though less strongly marked, and with a faint 

 groove (as in 8. granulosa, Billings) running from the apex of the glabella up to 

 but not across the anterior border. The thorax in these specimens is of four 

 segments, the last three of which, and possibly all, end in long spines, of which the 

 fourth is the strongest. The tails are somewhat indistinct, but are very similar to 

 those of the adult. These specimens are almost certainly young forms of the same 

 species, and they agree very closely with the specimens described by Dr. Groom 

 from Malvern under the name of Acanthapleurella grindrodi, the apparent absence 

 of the frontal limb in the Malvern specimens being probably the result of inrolling. 



"Affinities. — The Shineton form was described by Callaway in 1877 under the 

 name Gonophrys salopiensis, but seems to be identical with the Scandinavian species 

 8. pusilla. Brogger in 1882 pointed out its close relationship and probable identity 

 with Sars' Battus pusillus, which he accordingly named Conophrys pusilla and of 

 which he figured the head. This belongs, apparently, to an older individual than 

 the best of the Shineton specimens. It is larger and more trapezoidal in outline, 

 the differences being of the same kind as those between the large and small forms 

 described above. Moberg in 1890 figured both the head and tail of the species 

 and referred them to Billings' genus Shumardia. 



" S. miqiieli, Pompeckj, is very probably identical with 8. pusilla. The head- 

 shield in this form seems to be slightly distorted by cleavage ; and the small tail, 

 of the same shape as in S. pusilla, has only three segments and does not show the 

 rim, which may, however, be broken away. 



" The head-shield of 8. bottnica, Wiman, as Wiman himself observes, agrees 

 exactly with that of 8. jmsilla, and the single pygidium may well be that of a 

 young form of the same species. The head-shields of 8. celandica, Moberg, 1 and 

 S. dicksoni, Moberg, 2 are also almost identical, but while the tail of the former is 

 not far removed, that of the latter is widely different. 



1 Geol. For. Stockbolm Forh., vol. xxii (1900), p. 357, pi. xiv, figs. 4 — 6; Ceratopygeregionen 

 (1906), p. 79, pi. iv, figs. 7—9. 



2 Ceratopygeregionen (1906), p. 81, pi. iv, figs. 17—22. 



