1870.] 7 [Blisa. 



which covers the neck immediately in front of the dorsal fin. The 

 anterior, posterior and lateral margins of this plate are concave, 

 forming a pair of arm-like projections in front and behind. The 

 anterior arms partially clasp the plate, which I consider to be the 

 first dermoneural spine, and unite by suture with tne two parietals 

 and with the posterior angles of the suprascapular bones. The pos- 

 terior arms embrace the second dermoneural spine, and are firmly 

 united to the two lateral apophyses of the third interneural spine. 



Behind the plate just described rises the second dermoneural 

 spine ; but so modified that its true nature as a fin-ray is not at first 

 discernible. A smooth bony projection or keel rises from the top of 

 the second interneural spine, behind and somewhat beneath the 

 roof formed by the last described dermal plate. This keel, which 

 is somewhat sharply ridged and rounded anteriorly, supports the 

 second dermoneural spine. This spine is a short, triangular, forked 

 bone, anteriorly convex and posteriorly concave. When in posi- 

 tion it rides upon the keeled crest, and is hidden beneath the skin 

 surrounding the fin. Its posterior edges give attachments to liga- 

 ments connecting it with the third dermoneural spine, and a pair of 

 muscles extend from the arms of the rider to the anterior edge of the 

 articulating facet of the third interneural and to a portion of the 

 dermal plate above. From the anterior face of the rider, just above 

 the fork, arise two muscles which are attached to the inner surface 

 of the dermal plate immediately in front of the spine. These two 

 sets of muscles serve to move this spine up and down upon the keel, 

 by which the large spine behind it is raised and lowered as will be 

 described hereafter. 



The third neural spine rises above the second pair of para- 

 pophyses belonging to the enlarged centrum, and meets, and is 

 united to the interneural above, in the same manner as is the one 

 before it. It is also united to the neural spines before and behind 

 it. In fact, the neural and interneural spines of the second, third 

 and fourth vertebra are so soldered together as to form a wall almost 

 entirely dividing the space above the vertebrae. 



The lateral ridges of the third interneural spine expand supe- 

 riorly into two lateral apophyses modified into dermal plates. The 

 anterior edges of these apophyses are united to the posterior arms 

 of the plate in front of them ; posteriorly they extend downward and 

 outwardly, and terminate in an obtuse point. 



On each side of the fin the top of this third interneural spine is 



