1857.] EGERTON FISH-REMAINS FROM LUDLOW. 287 



assures us that, in that, the type of the genus, the cephahc buckler 

 was immediately succeeded by the scaly covering of the trunk, 

 without the intervention of a nuchal plate. Such a deviation from 

 the type is of too much importance to be considered a mere specific 

 character ; if substantiated, it implies generic distinction. At first 

 I imagined it might be an embryonic character, an idea somewhat 

 strengthened by the small size of the specimens. Should, how- 

 ever, the cranial shield of Cephalasjpis have resulted from the co- 

 alescence of several plates arising from distinct centres of ossifica- 

 tion, appreciable in the earlier stages of development, although sub- 

 sequently obliterated, the structure of all the plates should be ho- 

 mogeneous. This is not the case : the coarse fibrous character is 

 limited to the cranial shield, and the posterior plates seem to have 

 been composed of a compact material, more analogous in struc- 

 ture to true dermal plates. The specimens leave it in doubt 

 whether the post-cephalic plate was single or double. The occipital 

 crest extends backwards nearly to the extremity of the nuchal plate, 

 and in this the fibrous structure is evident, but there is no appear- 

 ance of any inosculation between this material and that constituting 

 the plates. It is therefore probable that this prolongation of the 

 occipital region of the shield gave attachment to a pair of plates, 

 one on either side, rather than that it constituted an integral portion of 

 a single plate. In selecting a generic title for this interesting form, 

 I am desirous of expressing the peculiar structure above described ; 

 I propose, therefore, to call the genus Auchenaspis. 



Of the two specimens submitted to examination, one is rather dis- 

 torted by pressure (fig. 5), the other retains its original figure (fig. 4). 

 I select the latter, therefore, for those details which have reference 

 to form. The outline of the shield anteriorly is nearly semicir- 

 cular. From the base of the occipital crest to the snout, it measures 

 8 millimetres ; of this the anterior portion from between the orbits to 

 the snout measures 5 milhmetres. The diameter across the base, a 

 little in advance of the lateral posterior angles, is 12 millimetres, or 

 nearly half an inch. Each nuchal plate is 5 millimetres in breadth 

 at its junction with the shield, and about 4 millimetres in length. The 

 posterior angles of the shield are short ; they project outwards and 

 backwards beyond the anterior margins of the nuchal plates. The 

 posterior border of the shield on either side the occipital crest is but 

 slightly curved ; it has a thickened margin, slightly bevelled poste- 

 riorly, for the attachment of the nuchal plates. In these respects 

 it closely resembles the corresponding parts of Cephalaspis Mur- 

 chisoni and C. ornatus. The texture of the bone is exceedingly 

 coarse, with the exception of the peripheral portion, which is 

 finer in grain, and appears to have been invested with an outer 

 layer of some thickness, forming a prominent border to the shield. 

 None of these structures bear any evidence of immaturity. The 

 nuchal plates are rather broader than long ; in form they very much 

 resemble the opercula of a Leptolepis or Philodophorus. The 

 under surface is traversed by a series of vascular grooves, di- 

 verging outwards from the inner anterior angles. The substance 



