360 University of California Puhlicaf ions in Zoology [Vol.16 



specialized than Hexanchus. Such certainly cannot be said of 

 Heptanchus maculatiis, for in this practically all traces of calcification 

 are absent from the central column, which represents only a slight 

 improvement over a notoehord of similar size. 



In the mid-body the central column assumes its simplest character. 

 Although there is considerable variation in the different specimens 

 dissected, our description will refer to what may be characterized as 

 the ordinary plan. In an immature specimen the mid-column (text- 

 fig. F) consists essentially of a heavy tube containing the constricted 

 notoehord; upon this cartilaginous tube clearly defined arches rest. 

 Immediately posterior to the rib-bearing segments the basiventrals 

 bend downward and on the forty-seventh join below to form the first 

 haemal arch (Ji.a.). From about the fiftieth to the fifty-fifth segment 

 of this region the basidorsal pieces extend entirely to the top of the 

 arch, no suprabasidorsals being present. Back of the region where 

 the high basidorsals occur (.56, text-fig. F) it is observed that the septa, 

 constricting the notoehord, are farther apart. Since each septum 

 strikes the middle of a segment of the central column it is clear that 

 the segments of the column in this area are much longer than in the 

 anterior regions. It will be observed that to each of these longer 

 segments two neural arches occur. This is the beginning of diplo- 

 spondyly, although the segments of the central column are not 

 themselves divided. Following this there is considerable irregularity 

 in the arches, but in general two types of basidorsals obtain, one of 

 which is high, the other is much lower. The higher of the two in 

 text-figure F is perforated by the ventral root of the nerve (f.v.) ; 

 the lower is imperforate. The higher is followed by an interdorsal 

 which is perforated by the foramen of the dorsal root-nerve (f.d.). 

 In the haemal or ventral arches the doubling of the plates in this 

 specimen begins unu.sually far forward. The first evidence of this is 

 in the forty-eighth segment. A regular doubling begins on the fiftieth 

 and intraventrals are added on the fifty-second segment. 



A segment of the colunui through the tail shows a typical 

 diplospondylous condition. In this region both the perforate and 

 imperforate basidorsals are practically uniform in size as well as are 

 the interdorsals. Above the interdorsals are the cartilages which sup- 

 port the dorsal lobe of the caudal fin. These are much more numerous 

 than the basidorsal and basiventral pieces, more than one hundred 

 being present back of the seventy-first vertebra. Below the seventy- 

 third segment in this region the haemal arches are well formed and 



