172 Proceedings of the Boyal Physical Society. 



is bispinose, one spine being much longer than the other, but 

 in neither case are the spines denticulated. Again, A. Grayce 

 does not appear to agree with any other described British 

 species with which I am acquainted, either by specimens or 

 figures, such as A. Parmndei, YletcheT ; A. hispinosa, M'Coy ; 

 A. Prightii, Murchison; A. coronatus, Salter; A. Jamesii, 

 Salter ; or A. Hughesii, Salter. In fact, the increased number 

 of spines round the pygidium separates A. Grayce from all the 

 foregoing forms, to say nothing of their denticulated char- 

 acter, except A. Mage and A. hystrix, where the spines are 

 twelve to fourteen in the one case, and twelve in the other, 

 and apparently all simple. All the specimens of pygidia 

 which I refer to A. Grayx, have constantly fifteen denticulated 

 spines. 



Leaving British species and passing to Bohemian Silurian 

 trilobites, we find there are several with which a comparison 

 may be made. M. Barrande has figured two species with the 

 pleural spines denticulated — Acidaspis Keyscrlingi, Barr.,* and 

 A. mira, Barr.f In the first of these the spines in question 

 are of a totally different form to those of A. Grayce, and the 

 thoracic axis is very much broader in proportion to the 

 general size of the trilobite. In A. mira the pleural spines 

 are of two kinds, one denticulated and the other not, both 

 attached to the same pleura. In neither are the spines of the 

 pygidium denticulated, but M. Barrande also figures at least 

 five species in which these particular spines are so, viz. : 



1. A. Verneuilii, Barr., loc. cit., t. 38, f. 1-9. 



2. A. Portlockii, „ ,, ,, f. 10-12. 



3. A. vesiculosa, ,, ,, ,> f- 13-21. 



4. A. tricornis, ,, ,, suppl., t. 8, f. 21. 



5. A. rara, ,, „ „ t. 12, f. 28. 



In the first three of these the form of the pygidium, and 

 the number, form, and arrangement of the spines into which 

 the limb of each is produced, are so different as to necessitate 

 little or no comparison. The ornamentation of these trilobites 

 consists of small granules or tubercles, both on the thorax 

 and pygidium ; on the other hand, on the only part of A. 

 Graym where the ornamentation is preserved, it is seen to be 



* Systeme Sil. Boh., i., t. 36, f. 10-22. t IMcl., t. 39, f. 1-11. 



