NECTURUS MACULATUS. 397 



one bearing the first haemal arch, vary considerably in their relation to costal elements. 

 The first of these vertebrae in many cases bears a small pointed rib with a capitular 

 attachment alone, but this seems to be more usual in cases in which the sacral vertebra is 

 the 19th. In one such case with the first haemal arch upon the 22d vertebra, the 20th 

 bore small ribs and upon the 21st appeared very minute capitular sockets, which, as this 

 was a macerated specimen, might have originally borne rib rudiments. There seems to 

 be much variation in these as well as in other particulars in this very variable region, but 

 the specimens examined have been too few in number to give any general conclusions. 



Study of Special Vertebrae. 



Atlas. — The main characteristics are the reduction in total length to about one half 

 that of a normal trunk vertebra, the peculiar shape of the centrum, the rudimentary con- 

 dition of the transverse process, and the conspicuous and projecting neural arch. The 

 posterior end of the centrum is normal but anteriorly it broadens rapidly to form a wide 

 wedge, the base of which is directed forwards and bears the two condyles. These are 

 slightly concave, elliptical surfaces, inclined a little outward, and meeting in the median 

 line where a small but conspicuous tubercle is formed that fits into a space upon the ven- 

 tral side of the skull between the two exoccipitals. The ventral aspect of the centrum is 

 somewhat hollowed and a little roughened for nruscular attachment, a peculiarity shared 

 by the 2d vertebra. The neural arch is large and conspicuous. Its sides are flattened 

 and obliquely set, and it ends in the mid-dorsal line in a rounded edge or keel, the poste- 

 rior end of which bears a small socket for the cartilaginous tip of the neural spine, as in 

 other vertebrae. The two posterior zygapophyses are large and heavy. The transverse 

 processes are rudimentary and very variable, even upon the two sides of the same 

 verte.bra. Their most constant parts are a usually bifurcated piece projecting obliquely 

 backwards in the same position as, and probably homologous with, the dorsal lamina of 

 the succeeding vertebrae ; and a much shorter spine, placed ventral to this and also jiro- 

 jecting backwards. The larger process is pierced by an obliquely directed foramen which 

 communicates with the neural canal and transmits the internal carotid artery and the first 

 pair of spinal nerves.^ Between this process and the short ventral spine there is a narrow 

 but very deep fossa. By the bifurcation of the original transverse process and the addi- 

 tion of the spine ventral to it, there are formed three backwardly directed processes which 



1 Hoffmann suggests that the phenomenon of a pair of spinal nerves boring through the substance of a vertebra in an 

 animal in which the nerve exits are normally intervertebral, is an indication that the "atlas" really represents two fused 

 vertebrae, the line of separation being marked by the nerve and its foramen. 



