560 MR. T. GOODEY ON THE SKELETAL [Mar. 15, 



number and distribution could be determined. The relation of 

 their foramina to the dorsalia presents a considerable difference 

 from the condition found on the other side of the tail. The total 

 number of neuromeres for the region in question is the same for 

 both sides, viz. forty-two ; but instead of the irregularly-disposed 

 foramina being separated by dorsal elements arranged in the order 

 of the numbers, 2, 2, 3, 9, and 11, we find the following numbers 

 of dorsalia separating them, 1, 2, 2, 2, and 14. From this it can 

 be seen that the regular diplospondylic condition has proceeded 

 one neuromere more posterior on this side than on the other, and 

 that the following three are also more regular than the corre- 

 sponding three of the other side. But there is no indication of a 

 return to the monospondylic condition. This very interesting 

 condition of heterospondyly is one which, so far as I have been 

 able to ascertain, has not been described for any other Selachian 

 fish. 



i. Summary of special features and comparison 

 with Notidanidse. 



In summarizing the leading characteristics of the vertebral 

 column of Chlamydoselachus, the following points may be men- 

 tioned : — First, the variety which it exhibits in the formation of 

 centra. At the extreme anterior end the constrictions form 

 cyclospondylic well-calcified centra. These may be followed by 

 smaller constrictions in a semi-calcified condition. In the trunk- 

 region are found the slightly constricted cylinders of the noto- 

 chord, representing the lowest of all the stages in centra- formation. 

 The main caudal region is characterized by the occurrence in it 

 of constrictions of two sizes, the larger more calcified ones lying 

 beneath the imperforate basidorsals, and the smaller less calcified 

 ones lying beneath the perforate basidorsals. This difference in 

 size gradually becomes lost as we proceed towards the tip of the 

 tail, the constrictions becoming equal in size concurrently with 

 the equalization in the size of the imperforate and perforate 

 basidorsals. This particular point of the occurrence of centra in 

 the tail-region is deserving of special emphasis, inasmuch as 

 three recent text-books of zoology give the uniform character 

 of the notochord and absence of centra in this region as a diag- 

 nostic feature of the Chlamydoselacbida?. The very pronounced 

 growth of the basidorsals and basiventrals around the chorda! 

 sheath in the main caudal region is also very noteworthy. The 

 great length of the diplospondylic region is of considerable 

 interest, extending as it does through thirty-eight neuromeres, 

 viz., from seventy to one-hundred and eight. The heterospondylic 

 portion of the tip of the tail is, so far as I am aware, unique in 

 Selachians. 



The points at which the calcified centra occur is perhaps 

 deserving of some mention. It seems that they are found where 

 there are the greatest demands made for strength. At the 

 anterior end, combined with the fusion of the vertebral column 



