THE OSTEOLOGY OF AMIURUS CATUS. 299 



which it articulates extends backwards as far as the posterior surface 

 of the defensive ray, which it supports. It is of a triangular shape, 

 broader behind than in front, and perforated by three foramina. The 

 two anterior are small, and situated one on either side of the middle 

 line, giving passage to the muscles which erect the small modified ray 

 lying in front of the defensive spine. The third is large, but is 

 divided into two parts by the extremity of the interspinal which sup- 

 ports the small modified ray just mentioned. 



This is shaped like an inverted U or a horse-shoe, and rests astride 

 of the extremity of the corresponding interspinal, the two limbs 

 passing down on either side through the large posterior foramen. 

 When erected it slides down over the anterior surface of the inter- 

 spinal, and the limbs then come into apposition with the preceding 

 expanded interspinal, so that it cannot be depressed until it is drawn 

 upwards again to its original position. The fixation is due to this 

 arrangement, the defensive ray being attached by a strong ligamen- 

 tous band to the extremity of this modified ray. The interspinal of 

 this horse-shoe ray is partly enclosed by the backwardly projecting 

 and strong spinous process of the fourth vertebra, and additional 

 strength is given by its union, by means of a thin osseous plate, to the 

 succeeding interspinal. Its extremity is smooth and is divided by a 

 slight transverse ridge into two parts, the posterior of which is a 

 continuation of the osseous plate between it and the succeeding inter- 

 spinal, originally formed in membrane, and, secondarily, united to the 

 bone developed round the cartilaginous interspinal. 



The succeeding ray is the defensive one. It is completely osseous, 

 slightly curved, and terminates in a sharp point. Its base is ex- 

 panded and presents three processes — two lateral, which rest on 

 either side on the horizontal plate already described, and a ventral 

 one which fits into a slight depression immediately behind the extrem- 

 ity of the interspinal of the preceding ray. Immediately above this 

 ventral process is a perforation, which, when the ray is erected, re- 

 ceives the extremity of the preceding interspinal, and above this per- 

 foration is a rough surface for the attachment of the ligament by 

 which the ray is united to the preceding one. The interspinal cor- 

 responding to this ray is situated in the cleft extremity of the spin- 

 ous process of the fifth vertebra, and is united with the preceding 

 interspinal by the thin plate already described ; above it expands 

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