196 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



present description is based, which measured 52cm., the 

 base of the first dorsal fin ray was only 6mm. from the 

 posterior narial aperture of the eyeless side, i.e., well in 

 front of the cavity of the brain. 



As in the paired and caudal fins all the fin rays con- 

 sist of two pieces. Each fin ray is connected with two 

 further skeletal pieces termed by Cope,* Baur,t and 

 Smith Woodward^ the baseost and axonost, by T. J. 

 Parker§ pterygiophores, and by Bridgell radial elements. 

 The axonost has hitherto been called the interspinous 

 bone, on account of its position between two neural spines. 

 The terms baseost and axonost are adopted in this work, 

 and the precise connections between these two elements and 

 the two pieces forming a fin ray will be described below. In 

 the meantime it may be remarked that usually one or two 

 axonosts are found between two adjacent neural spines in 

 the dorsal and anal fins of the Plaice, whilst the baseost 

 is alwa}'S situated between and attached to the heads of 

 two adjacent axonosts. The two halves of the fin ray 

 diverge proximally and tightly clasp the baseost (cp. fig. 

 16). This mechanism was first described by T. J. Parker in 

 Regalecus, and has since been described in the Plaice by 

 Bridge. Each axonost, even in old specimens, consists 

 usually of a bony cylinder filled with cartilage. The head 

 is always hollow, and is filled up with a triangular plug 

 of cartilage (fig. 16). 



The first dorsal fin ray has no baseost, but its halves 



diverge as usual and embrace the head of the first axonost 



with only a small sub-cartilaginous pad between. It is 



also asymmetrical, the ocular half being slightly the 



longer. 



* American Nat., 1800. f Jour. Morph., vol. iii. 



J Catalogue Fossil Fishes, British Museum, and Vertebrate 



Palaeontology. 



S Trans. Zool. Soc. vol. xii Journ, Linnean Soc, vol, xxv 



