BRITISH FOSSILS. 



9 



Fig. 7. Eye, magnified. 



Fig. 7*. Do., still more highly magnified ; the facets are convex ; and at a, one is in 

 its original condition ; the pits on the others are due to wear. 



Fig. 8. Two thorax joints of Aymestry specimen (fig. 1) ; at a and b, the curious 

 tubercles at the fulcral point are seen ; c, is the outer spinose portion ; 

 d, the inner bilobed part ; they are separated by a furrow, /. 



Fig. 9. Specimen from Nash Scar, Presteign, collection of J. E. Davis, Esq. This is 

 an internal cast, and shows the outer tubercle e, and the furrow g, more 

 strongly than in fig. 8, which has the crust on. 



Fig. 10. Under view of cheek from the same specimen (as fig. 9) ; it has an unusual 

 swelling above the facial suture a. 



Fig. 11. Perfect hypostome (collection Geol. Surv.), from Dormington Wood, Wool- 

 hope ; a, the lateral ascending processes ; b, the marginal wings. 



Fig. 12. Side view of do. ; the incurved triangular plates are shown at 6, the lateral notch 

 at c. 



Fig. 1 3. Under view ; a, a, the " ascending processes," which are attached to the under 

 surface of the glabella at its sides ; b, b, the incurved triangular lateral plates, 

 possibly for the attachment of muscles ; c, the hollow space under the 

 ascending processes, answering to the lateral notch in fig. 12. 



Fig. 14. Outline of the largest hypostome we have seen, from the Lower Silurian lime- 

 stone of Kildare, Ireland ; the letters are the same as in fig. 1 1 . 



Fig. 15. Lateral view of the same. 



Fig. 16. Var. j8, centralis, from the Wenlock strata of Nelson's Tower Wood, Llandeilo. 



Remarks on the Genus» 



It seems necessary to contend for the generic name adopted here, because a rigid 

 adherence to priority would compel us to relinquish a name now familiar to naturalists, and 

 bestowed by Beyrich on a group which he had carefully investigated and fully described. 

 Now that Hall has given such excellent figures of Ceraurus, we know perfectly well 

 what was meant by the obscure and imperfect plaster cast published by Green under that 

 name. But the original description was scarcely more than sufficient to indicate that it 

 was a trilobite, and consequently it has been referred with doubt to various genera by Boeck, 

 Beyrich, Loven, Portlock, and Burmeister. A genus so ill constructed and imperfectly 

 described, can have no authority ; and it would be unjust to substitute such names for 

 those given by the first real describers. The same rule we think fully justifies us in 

 rejecting Zethus of Pander, a name lately revived by Dr. Volborth*; for the genus as 

 constituted by Zander consists of two species, to either of which the meagre and incorrect 

 description will apply ; the first"tof these being, by Dr. Volborth's own admission, a 

 species of Cheirurus, the second a Cybele^ He would restrict the name to the latter ; but 

 custom and the opinion of naturalists in general would point in doubtful cases like this to 

 the first as the typical species, and we should then have to apply Zethus to all we now call 

 Cheirurus ; more especially as it was the Cheirurus only of which Pander knew the entire 

 body. He describes it as having 16 ribs in thorax and tail together, the segments of the 

 tail being free like those of the thorax ; this is untrue for either genus ; and he denies any 

 trace of eyes. Of the Cybele, a fragment only is figured, and Pander even doubts whether 

 it belongs to the genus, so that he evidently intended the first for his type ; and had either 

 his figure or description been intelligible, or had he referred to Sternberg's or Dalman's 

 species as cognate, his name ought to have been retained. But we believe the right of 

 priority of name, rather than that of description, cannot with advantage be so rigidly 

 enforced, and we accept Cheirurus as the first intelligible description, as well as the 

 clear definition of a remarkable group. With regard to the affinities of the genus, we have 



* Transactions of the Koyal Mineralogical Society of Petersburgh (1847.) 



