1910.] AXATOMY OF THE PRILLED SHARK. 559 



of the nerve foramina, which perforate the dorsalia. Anterior to 

 the portion figured, the caudal region is typically diplospondylic, 

 as already described. The ventral foramina are, throughout the 

 vertebral column, larger than the dorsal ones. At the com- 

 mencement of the figure, on the left-hand side can be seen a 

 basidorsal which carries a nerve foramen. The succeeding inter- 

 dorsal is imperforate, the dorsal and ventral roots of the spinal 

 nerve having apparently united or approximated very closely 

 together so that one foramen will transmit both. This condition 

 also holds for the succeeding spinal nerves. In counting the 

 number of dorsal elements separating the foramina depicted in 

 fig. 15, it is seen that between the first foramen and the following 

 one there are two doi-sal elements. Between the second and third 

 there are also two ; between third and fourth, three ; between 

 fourth and fifth, nine ; between fifth and sixth, eleven ; and 

 between this and the end of the tail there are thirty elements 

 without a single perf oration. This arrangement of the foramina 

 in relation to the number of dorsalia is obviously quite irregular, 

 and so far from being in accord with Ridewood's (28) and Maver's 

 (22) contention that the terminal region of the vertebral column 

 is monospondylic, it shows that in Ghlamydoselachus, at any rate, 

 the neural apertures are so irregular in arrangement that this 

 particular region may quite well be termed ' heterospondylic' The 

 musculature of the region in question is very much reduced, and 

 we should scarcely expect to find so perfect a nerve-supply as is 

 found more anteriorly. Moreover, with the irregularity in the 

 shape and size of the arcualia and their non -segmental arrange- 

 ment relatively to one another, it is difficult to imagine how a 

 monospondylic condition could obtain here. 



It seemed desirable to ascertain in what relation the myomeres 

 of the tail stand to the neuromeres. In order to do this the skin 

 was taken oft' from the other side of the tail and posterior portion 

 of the trunk, so as to reveal the myomeres with their separating 

 myocommata. On examining the limits of the myomeres it was 

 seen that each one in the trunk corresponds in extent with a 

 monospondylic neuromere. In the main caudal region each 

 myomere is equal in extent with a diplospondylic neuromere. 

 A determination was next made of the number of myomeres from 

 the beginning of the diplospondylic region to the point where the 

 distinction between the separate myocommata is lost, i. e. within 

 five centimetres of the extreme tip of the tail. This number is 

 forty-two. The number of neuromeres was next determined for 

 the same region, and this also is forty-two. Thus the number of 

 myomeres and neuromeres is the same in the tail, and each 

 irregular or heterospondylic neuromere of region " 4 " has its 

 corresponding myomere. 



The number of neuromeres for the side of the tail on which 

 the myomeres were counted was next determined. After care- 

 fully removing the muscular tissue, the spinal nerves were left, 

 and by examining these through a dissecting microscope their 



