332 Mr. C. T. Regan on the 



baseosts being either small or absent. Caudal abbreviate 

 heterocercal, homocercal, or diphycercal. Paired fins usually 

 not lobate. Pectoral metapterygium sometimes well deve- 

 loped and serving for the articulation of the posterior 

 baseosts, more often reduced and apparently forming the first 

 of the baseost series. Ventral with the dermal rays directly 

 attached to a single basal bone, the baseosts rudimentary or 

 absent. Hyostylic. Hyomandibular with a posterior process 

 for the articulation of the operculum ; symplectic ossified and 

 usually suturally united to the quadrate. Branchiostegal 

 rays attached to the epihyal and ceratohyal. No paired gular 

 plates. Clavicle not distinct from the cleithrum. Vertebral 

 column variously developed. No communication between 

 pericardium and coelom. 



It need hardly be pointed out here that I cannot expect the 

 characters used in the above ordinal definitions to prove 

 constant in every case. Experience shows that, however 

 well defined groups may seem to be, as our knowledge of 

 them becomes more complete annectent forms come to light, 

 and it is self-evident that if we were acquainted with all the 

 forms which have existed we should have a perfect phylo- 

 genetic arrangement, but no division into groups. Conse- 

 quently the generalizations which I have made may or may 

 not be applicable to those unsatisfactorily known extinct 

 forms (<?. g. Oatopteridse) which can only be provisionally 

 assigned to a position in the system. 



ClIONDEOSTEI. 



The Chondrostei, which have been regarded by some as 

 modified Crossopterygii, are undoubtedly the most generalized 

 of all Teleostomi. 



The ventral fins of Polyodon, Acipenser, and Scaphi- 

 rhynchus have been well described and figured by Thacher* 

 in 1877, and also by DavidofTf in 1879, the former of whom 

 regarded their structure as most important evidence of the 

 truth of his theory of the similar origin of the median and 

 paired fins. This view was also accepted by Bridge J, who, 

 in 1878, referring to Polyodon, wrote : — " The evident 

 formation of the ventral fins by the coalescence of a series of 



* Tr. Connect. Ac. iv. 1877, p. 234, pis. i. & ii. 



t Morph. Jahrb. v. 1879, p. 450, pi. xxviii. See also Wiedersheiin, 

 • Gliedmassenskelett,' p. GO (1892). 



| Phil. Trans, clxix. 1878, pp. 683-734. 



