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



apparent. Each of these occurs beneath a basidorsal, anil extends 

 between two consecutive segmentation marks on the exterior of 

 the chordal sheath. Each takes the form of a bulging inward 

 of the sheath, so tha.t a slightly pinched-in cylinder is formed. 



The regions described thus far are typically monospondylic, 

 i. e. each neuromere is made up of one of each of the vertebral 

 elements, one basidorsal, one interdorsal, one suprabasi dorsal, 

 one basi ventral, and one interventral The foramina for the 

 spinal nerves do not occur between the dorsalia, but are actual 

 perforations of the basidorsals and interdorsals. In the mono- 

 spondylic regions each basidorsal transmits a foramen for a 

 ventral root and each interdorsal one for a dorsal root. The ventral 

 root foramina are larger than the dorsal ones. 



Bibs. These are small, thin, cartilaginous pieces segmented off* 

 from the basiventrals, with which they are continuous. They 

 occur in regions 1 and 2, and extend from the eighth to the sixty- 

 fourth neufomeres inclusive. At their posterior end they reach 

 a point on the vertebral column a short distance anterior to 

 the level of the cloaca, where they terminate abruptly, having 

 apparently diminished but very little in size. More posterior to 

 this point the basiventrals begin to grow downward, and gradually 

 assume the form of wedge-shaped pieces which afterwards fuse 

 beneath the haemal canal and thus give rise to the haemal spines. 



Region 3. — At the seventieth neuromere we get the transition 

 from the monospondylic to the diplospondylic condition taking 

 place (see fig. 12). As represented in the figure, the latter con- 

 dition appears to be brought about by the segmenting off of a 

 small basidorsal from the anterior side of a typical monospondylous 

 one. By this means each single large basidorsal gives rise to two 

 smaller ones, and between these there is inserted a small inter- 

 dorsal. The small basidorsals have narrow suprabasidorsals seg- 

 mented off' from their apices as thin wedge-shaped pieces. A 

 ventral root foramen perforates the posterior one of each pair 

 of diplospondylous basidorsals, whilst the succeeding interdorsal 

 transmits a dorsal root foramen. In this way we have the 

 typical diplospondylic condition of vertebrae brought about, and 

 this arrangement obtains to a point about eleven centimetres 

 from the tip of the tail. At the seventy-second neuromere, as 

 shown in fig. 12, we find the monospondylic condition again 

 occurring, apparently as a reversion to the more primitive stage 

 in development. Instead of finding two of each of the arcualia 

 we only have a single large basidorsal with a ventral root foramen 

 near its posterior edge, followed by a single large interdorsal. 

 However, on the lower side of the notochord there are two basi- 

 ventrals and interventrals, thus indicating that although the 

 dorsalia have not been segmented into the double condition, yet 

 this has occurred in the ventralia. This single neuromere is of 

 interest, because it seems to indicate that the diplospondylic con- 

 dition is the secondary one, arising by segmentation of the parts 

 which go to form the more primitive monospondylic condition. 



