COEEESPONDENCE. 



451 



Itestoration of Fteraspis. 



My dear Sir, — I have read the Eev, 

 H. Mitchell's paper on " The Itestora- 

 tion of Pteraspis " with great interest, 

 but the conclusions he draws from his 

 Scotch specimens are, I think, by no 

 means borne out by examples obtained 

 from English localities. I enclose you 

 two sketches of Pteraspids from Crad- 

 ley, in Herefordshire : one, in my own 

 collection, showing the anterior portion 

 of the shield ; the other, exhibiting the 

 posterior portion and surface markings, 

 in the possession of Mr. Gregory, of 

 Golden Square. These two sketches 

 will be sufficient to show that the shield, 

 as drawn by Mr. Mitchell, is wanting in 

 some important particidars ; the true 

 form I believe to be that given in fig. 3. 

 Mr. Mitchell seems to be under the im- 

 pression that the restoration of Pteraspis 

 has never before been attempted, al- 

 though first-rate specimens have been 

 in our museums and ])rivate collections 

 for some years. Professor Huxley, in 

 vol. xvii. of Geol. Soc. Journal, has given 

 a diagram of a restored Pteraspis, which 

 is copied in fig. 3 ; he has likewise re- 

 ferred to the subject in British Asso- 

 ciation Heports, 1858, and has further 

 written a detailed account of the microscopic structure of the test in vol. 

 xiv. of the Journal. The references in fig. 3 are as follows : — a is the snout 

 or rostrum, united with h, the shield-like 

 disk ; c c are the lateral cornua attached to 

 the disk ; e is the posterior spine, and/y are 

 the orbits or perhaps the nasal apertures. 

 The corresponding parts can be easily found 

 in the sketches I have given of the fossil 

 Pteraspids. I may just mention here, that 

 the bonelike test of these fish is composed of 

 three layers, — an external, finely striated 

 layer, a middle cancellated layer composed 

 of polygonal cells, and a third internal 

 layer of a laminated nacreous substance. 

 The specimen drawn in fig. 2 shows the 

 external layer remarkably well ; in fig. 1, 

 parts of the middle cancellated layer may 

 be seen ; the rest of the test, which is pre- 

 served, being the internal nacreous layer. 

 I remain, dear Sir, yours truly, 



E. li. Lankester. 



8, Savile Roto, Nov. 9. 



P.S. — You will observe some minute in- 

 dentations drawn in fig. 2, on the surface Fig. 2. 



Tier. 1. 



