456 MR. C. TATE REGAN ON THE 



tail being shortened so that the fin became terminal rather than 

 ventral. The use of the jaws for crushing shell-fish instead of 

 seizing fishes may account for the firm union of the prsemaxillaries 

 with the frontals. The lower jaw of the Palseoniscids appears 

 to have consisted of the same four bones as that of Folypterus, 

 namely dentary, angular, articular, and preearticular ; that of 

 the Semionotids is shorter, deeper, and stronger, and the 

 additional articular ossifications (as seen in Amia) may simply be 

 due to the increased size and complexity of its articulation, whilst 

 the intra-dentaries may be regarded merely as plates developed to 

 support teeth. The " supra-angular " may be an anterior bone of 

 the outer cireumorbital series which has adhered to the coronoid 

 process, in the same way that a bone of the temporal series has 

 become attached to the hyomandibular in Polypterus\ and I believe 

 that the supplemental maxillary had a similar origin. The small 

 size of the mouth involved a change in sha,pe and direction of the 

 praeoperculum and hyomandibular ; the prseoperculum, which in 

 the Palseoniscidse appears to have been simply a plate covering 

 the cheek behind the circumorbitals, now became used to 

 strengthen the suspensorium, a necessity on account of the 

 forward position of the articulation of the lower jaw ; the lower 

 end of the elongated hyomandibular ossified as the symplectic, 

 which may have been originally developed either, as I am inclined 

 to think, for the articulation of the lower jaw, or perhaps only 

 to support the quadrate and connect it with the prseoperculum ; 

 the original hyomandibular ossification developed a knob for 

 . articulation with the operculum ; the upper ossification in the 

 cerato-hyal appears to have been developed for the attachment 

 of the interhyal, which seems to be an ossified ligament. It is 

 obvious that the character of the respiration would be greatly 

 changed, the movements being smaller, and I think that a better 

 regulation of these would result from the definite articulation of 

 the operculum with the hyomandibular ; moreover, the hyomandi- 

 bular being now but little movable, movements of the operculum 

 became of more importance. The interoperculam is a new bone 

 of some interest; it would almost seem as if the suboperculum, 

 retaining its attachment to the posterior end of the lower jaw, 

 had been elongated and then fractured, the posterior part thus 

 retaining its freedom of movement, whilst the anterior — the 

 interoperculum~was overlapped by the horizontal limb of the 

 preeoperculum. Finally, one may suspect that the disappearance 

 of the clavicles is in some way connected with the other read- 

 justments in this region. 



The Semionotidte appear to have developed in various direc- 

 tions, giving rise for example to the remarkable Pycnodontidse, 

 which show many resemblances to our raodena Plectognaths ; and 

 through the Eugnathidse to the Amiidse and to the Pachycormidfe, 

 which parallel the Scombroids. The development of a large- 

 mouthed piscivorous type appears to have taken place in two 



