50 Proceedings of the Royal Physical Society. 



cranium of a deer or sheep in the Black Forest, which im- 

 pressed them with the resemblance the cranium bore to a 

 series of vertebrae more or less fused together, the theory of 

 a unity of organisation, traceable throughout all the verte- 

 brate and articulate classes, has been wrought out down- 

 wards to the lowest Eozoa or Acrita. The main object of 

 this communication is to point out the homologies which 

 appear in the human cranium, as the first part of the subject, 

 and especially, to determine the true homologies of the dif- 

 ferent types from man to the lowest ichthyia or osseous fishes, 

 and afterwards those analogous homologies of Scleroderma 

 among the Annulozoa and Arthrozoa (the Articulata. — Cuv.) 



A few changes in the usual terms of the anatomical school 

 will be required for the clear elucidation of this important 

 subject. Skelon will be substituted for Endoskeleton, and 

 the different segments will be styled Skelotomes. Instead 

 of Exoskeleton or Scleroderm,/SWercw will be used, and Sclero- 

 tome for any segment seen in the Arthrozoa and Annulo- 

 zoa. Somatome will include a segment of both skelon and 

 scleron, together with the visceral or splanchnoskelo7i, and 

 the soft parts connected with them. These terms are 

 slightly modified from those proposed by Professor Goodsir. 



The characteristic difference distinguishing the verte- 

 bralia from all other members of the zoological scale is 

 drawn from the existence of a central stem or kaidon, usually 

 styled the Spinal or Vertebral column. I restrict this term 

 to the stem formed by the centra or bodies of the vertebra? 

 continued through the basicranium, having sometimes a 

 mesospine on its ventral aspect. On its dorsal aspect, the 

 perineural tunnel protecting the myelon or spinal marrow, 

 is formed by the coalescence of the neur-arcs on each side, 

 firmly anchylosed to the individual skelotome of the kaulon, 

 and extending backward to unite in the neural spine. 



These Neur-arcs are formed of three distinct parts, like 

 all other laminae — 



1. The Pedicle fused into the vertebrae, and forming part 

 of the aperture for transmitting the spinal nerves (trous des 

 conjugaison). 



2. The Lamella, generally the largest part, with the tubercle 



