856 DR RAMSAY H. TRAQUAIR ON FOSSIL FISHES COLLECTED BY THE 



obsolete guesses as to a true zoological (we should now call it a genetic) affinity between 

 the ancient plated fishes of the Silurian and Devonian epochs and Arthropoda. It will 

 be remembered that nearly ten years ago Prof. Patten compared the sutures and other 

 markings on the head of a Trilobite with those on the cranial shields of Pterichthys and 

 Bothriolepis, though unsuccessfully, as he had unfortunately taken his figures from old 

 restorations, in which sutures and sensory grooves were confounded. He now (xxii.) 

 finds a strange similarity between the microscopic characters of certain chitinous 

 trabecular structures "underlying the external chitinous covering of the body" of 

 Limulus and those of the plates of the carapace of Pteraspis. These trabecules contain 

 in their centres, or cores, minute cavities, which he compares to bone-lacunas ; and each 

 of these sends off a delicate tubule or canaliculus vertically to the surface. " When 

 the chitinous network forms a rather thin, layer, as in the eye region and elsewhere on 

 the thoracic shield, the innermost trabeculas unite to form a nearly continuous layer, 

 perforated by pores, that lead into the irregular sinuses above them." This he compares 

 with the basal layer of Pteraspis, while the layer above, " crossed in various directions 

 by the chitinous trabecular containing the lacunas," he divides into two, of which the 

 lower corresponds to the cancellated, the upper to the reticulated layer in the same 

 fish-plates. Lastly, the " thick outer cuticula " he compares to the outer layer of 

 Pteraspis, the most superficial and colourless stratum representing the ganoine, 

 while he evidently looks upon the deeper part which is permeated by " innumerable 

 canalicular (pore canals of authors) " as the dentine or kosmine layer. He con- 

 cludes the descriptive part of his paper by stating that " the only animals 

 known to show such an exoskeleton as Limulus are some of the remarkable 

 fishes known as Cephalaspidas," under which designation he includes also the 

 Pteraspidas. 



I have not myself examined the dermal structure of Limulus microscopically, but 

 I have read Prof. Patten's paper very carefully, and must own that I fail to see, more 

 especially in his figures, such a correspondence between the structure descrihed in 

 Limulus and the structure of the plates of Pteraspis as would warrant us in supposing 

 that the two forms were " genetically related." Pteraspis has no bone-lacunas, hut 

 here Prof. Patten calls in the aid of Cephalaspis, in which, however, the lacunas 

 are quite different from the appearances figured as such in Limulus. Nor do I see 

 anything in Prof. Patten's figures having the slightest resemblance to the dentine or 

 kosmine layer of Pteraspis. Prof. Patten says — " As to its peculiar surface orna- 

 mentation, the shield of Pteraspis is exceptional among the Cephalaspidas, and need not 

 at present be considered." But it is here where the gist of the matter comes in. The 

 crenulated ridges of Pteraspis obviously consist of shagreen-bodies, like those of 

 Thelodus, but run together in lines, and the derivation of the family from an Elasmo- 

 branch source is indicated in a way, which superficial resemblances to other groups 

 cannot in any way touch. 



To sum up, then, I must consider the Heterostraci to be of Elasmobranch deriva- 



