THE OSTEOLOGY OF AMIURUS CATUS. 295 



jecting process from that plate. The fifth is of the normal type, all 

 its parts being present, but its body is united anteriorly with that of 

 the fom-th. The bodies of the 2nd-5th are deeply grooved below for 

 the reception of the aorta. 



The bodies of the succeeding vertebrae as far back as the com- 

 mencement of the tail fin are all similar in appearance. They are 

 of the usual piscine ainphicoelous type, but they are very much 

 flattened at the centre of their length from above downwards, and a 

 strong longitudinal ridge extends along the lateral surface of each, 

 increasing the appearance of flattening. In the adult the bodies, as 

 well as the arches, are thoroughly ossified, no notachord remaining 

 in the centre of the bodies. In a stage incompletely ossified it may 

 be seen that the notachord is contracted very much vertebrally, ex- 

 panding rather suddenly as one approaches either exti-emity of the 

 body, and resuming its full uncontracted size. The lateral ridge 

 seems to be formed by an extension of the ossification into the adher- 

 ent connective tissue along the lateral line of the column. On the 

 upper and lower surfaces of each centrum, on either side of the 

 middle line, is a ridge, so that viewed laterally the vertebrae do not 

 appear extraordinarily flattened. Posteriorly in each vertebra, i.e., 

 between the attachment of successive arches, these ridges increase in 

 height, thus forming a protection for the spinal cord or aorta between 

 the arches. 



The arches are completely ossified, and are firmly anchylosed with 

 the bodies. They unite with the anterior portions of the bodies 

 above and below, enclosing in either case the spinal cord or the 

 aorta. In the more anterior dorsal arches the anterior elevations of 

 the dorsal longitudinal ridges of the centra articulate with the 

 posterior border of the preceding arches, but posteriorly no such 

 articulations obtain. All the dorsal arches, and the haemal arches 

 also in the tail region, are surmounted with long backwardly directed 

 spinous processes ; those of the 5th-9th dorsal vertebrae inclusive being 

 bifid for the reception of the interspinalia of the dorsal fin. The 

 majority of the vertebrae of the trunk region have their lower 

 arches projecting at right angles from the centrum, forming the 

 transverse processes ; with the 6th-14th of these ribs (ossifications of 

 intermuscular septa) articulate, the upper surfaces of their proximal 

 portions being in contact with the under surface of the distal ex- 

 tremities of the transverse processes. The last two vertebrae of the 



