558 PROF. E. W. CLAYPOLE ON THE STEUCTTJEE 



and both would most likely fall into the genus Palceaspis as here 

 amended, no ver}^ important difference then remaining so far as 

 they are yet understood. 



Yet farther in this direction may be quoted the Diplaspis of 

 Mr. Matthew, in which he represents in his figure what appear to be 

 two such lateral plates on each side of his fish. His specimen is so 

 dam aged that this point is left doubtful, and Mr. A. Smith Woodward 

 says in his ' Catalogue ' that he prefers to consider them as the 

 result of fracture till further evidence is forthcoming. But if in 

 reality Diplaspis possessed both an antero- and a postero-lateral 

 plate, this fact would greatly strengthen the argument that such a 

 structure was not uncommon in the family. The place of meeting 

 of these three plates would then furnish a critical point to which an 

 organ of motion, if one existed, might naturally be attached. 



It may further possibly turn out that this division' of the single 

 lateral plate of Cyathaspis of Europe into two may be a character 

 of all the American Pteraspidians, and the difierent genera (if 

 more than one exist) must be characterized by other features. 

 We are unable on this point to form an opinion regarding Kunth's 

 specimen. 



Eecurring for a moment to Kunth's figures, it is not easy to 

 understand the relations of the other parts of the fossil there 

 represented. But the fragment shown crosswise and behind the 

 dorsal plate is certainly sufficient to justify the above suggestion. 

 Evidently the dorsal shield extended farther, and possibly much 

 farther, backward than the fragment shown in the figure indicates ; 

 while the lateral plate shown out of position may readily be 

 supposed to have extended from the bevelled edge of the Scaphaspid 

 plate backward, and to have gradually tapered off at the end of the 

 Pteraspid shield. 



It is not a little surprising that in his attempted restoration 

 Dr. Kunth has entirely omitted this lateral plate. He evidently 

 regarded it as belonging elsewhere than between the two main 

 shields, and hence, though it is represented in the end view, it is not 

 shown in his fig. 5. 



Regarding the. medley of fragments lying behind the large shield, 

 we cannot from' Kunth's figure form any definite idea of their 

 nature and relations. Such fragments are always difficult of inter- 

 pretation, and the pencil of the learned author may have been 

 unconsciously swayed by his belief that be was representing a 

 trilobitic crustacean.^ Whatever their appearance may be they 



^ That Dr. Xunth was somewhat biassed by his prejudice in favour of a 

 crustacean affinity for his fossil appears almost certain when we read his 

 remarks on the wider bearings of his subject towards the end of his paper. He 

 saw of course that his conclusions regarding his own species, which he named 

 Cyathas'pis integer, would logically be applied to Pteraspis in consequence of 

 their manifest affinity. But Lankester had figured a Pteraspis with a few 

 evident scales behind the dorsal shield. This specimen, the only one yet known 

 showing any similar parts, established of course the ichthyic nature of Pteraspis. 

 With a view of invalidating the strength of this oppo?ing testimony Dr. Kunth 

 remarks : — " Schwer zu enischeiden ist es, ob die Gattung Pteraspis im Sinne 

 Lankester's hierher zu stellen ist. Lankester hat ein Stiiek Schild von Pteraspis 



