50 PEOE. E. W. CLAYPOLE ON PTEEASPIDIAN ETSH IN 



1. The close Zoological Affinity of the Fossils. ' 



In entering upon this part of my subject, both duty and pleasure 

 make it just to express my thanks to several friends for the very 

 kind assistance which they have afforded me in obtaining specimens 

 of the English fossils for comparison. In the first place my thanks 

 are due to my old and valued friend C. W. Peach, Esq., of Edin- 

 burgh, for offering me specimens of Pteraspis, collected by himself 

 many years ago in Cornv^all, and of Cephalaspis, from the cabinet 

 of J. Powrie, Esq.. In the second place, I must express my in- 

 debtedness to my friend and former pupil, Mr. T. Stock, of the 

 Museum of Science and Art, Edinburgh, and to Mr. B. N". Peach, 

 of the Geological Survey of Scotland, for obtaining and forwarding 

 these and some other material of which I had need. Without their 

 kind aid this part of the subject could not have been made complete. 



It is scarcely necessary to dwell long upon the outward form and 

 appearance of the fossils ; they closely resemble the Ludlow species 

 in these respects, being oval in shape and having a curved surface 

 marked with a delicate striation (figs. 7 & 8, pp. 62, 63). But the 

 details are different in several points. I have seen no indications 

 of the medial posterior spine which terminates the shield of some 

 of the Ludlow forms ; the striation of the surface also shows marked 

 distinction. Instead of flowing, as represented in Prof. Lankester's 

 monograph, concentrically round an excentric point, like that on 

 the shell of a Lingula or a Discina, it is very irregular. Generally 

 speaking, several lines run round the margin, curving sharply 

 inwards at a point which probably indicates the orbit. From these 

 lines others radiate very irregularly towards one or several centres, 

 sweeping over the surface, separating and anastomosing, and form- 

 ing a beautiful and delicate tracery quite distinct from anything 

 represented on Prof. Lankester's figures (see fig. 7). 



Professor Lankester has separated the fossils which form the subject 

 of his memoir into three divisions, based on the number of pieces of 

 which the shield consists. These divisions are : — 



1. Scaphaspis. Scutum simplex, ovale. 



2. Cyathasjpis. Scutum in quatuor partes divisum, ovale. 



3. Pterasp)is. Scutum in septem partes divisum, sagittiforme. 



Of these three, Scaphaspis is the oldest and simplest form, and, 

 with one doubtful exception, is the only one yet obtained from the 

 English Silurian rocks. It is quite in harmony with these facts 

 that the same form should prevail among the Pennsylvanian fossils. 

 Most of them consist of a single piece, but some present appearances 

 which suggest the probability that they possessed other plates in 

 front and laterally. If this prove to be the case, they will approach 

 the genus Cyathaspis of Lankester. 



In regard to the microscopic structure of these fossils and their 

 resemblances and differences as compared with those of the English 

 Ludlow rocks, I must confess my indebtedness to the figures of 



