242 IOWA GEOLOGICAL SURVEY 



The same author remarks further, iu his "Outlines of Ver- 

 tebrate Palaeontology," that the group "is also specialized in 

 (i.) the fusion of the bones of the pterygo-quadrate arcade, (ii.) 

 the reduction of the infradentaries to one, (iii.) the reduction 

 of the opercular apparatus to the operculum on each side and 

 a pair of gular plates, (iv.) the loss of the baseosts in the an- 

 terior dorsal fin, and (v.) the ossification of the air-bladder." 

 The typical genus Coelacanthus ranges from the Devonian to 

 the Permian inclusive ; Undina is the best-known Jurassic genus ; 

 Diplurus, with greatly elongated caudal pedicle, is limited to the 

 Trias of this country; and Macropoma occurs in the English 

 and Bohemian Upper Cretaceous. 



Family COELACANTHIDAE. 



The principal diagnostic features of this family are thus sum- 

 marized by Professor Bridge in the volume on Fishes in the 

 Cambridge Natural History: 



"Scales cycloid. Paired fins obtusely lobate. Tail symmet- 

 rical but apparently gephyrocercal, usually with a protruding- 

 axial vestige of the disappearing terminal part of the tail and 

 of the proper caudal fin. Eadialia of the functional caudal lobes 

 agree in number with the contiguous neural and haemal arches 

 and dermal fin-rays (the diagnostic feature of Smith "Wood- 

 ward's Actinistia). Proximal radials of the dorsal and anal 

 fins fused into a single, internally forked basipterygium in each 

 fin. Teeth simple. Vertebral column acentrous. The skull 

 presents several interesting features. The hyomandibular and 

 the palato-quadrate bar, for example, are fused on each side 

 into a continuous triangular bone, articulating with the cranium 

 above and with the lower jaw below. The opercular skeleton is 

 reduced to an operculum and two jugular plates. A very singu- 

 lar modification in these fishes is the ossification of the walls of 

 the air-bladder, a structural modification which has no parallel 

 in Fishes except in certain Teleosts (Siluridae and Cyprinidae) 

 in which the organ becomes encapsuled by bone owing to the 

 partial ossification of its walls." 



This family, first proposed by Agassiz in the second volume of 

 his Poissons Fossiles, (p. 168, 1844), and afterwards greatly re- 

 stricted by Huxley in two important memoirs of the British 

 Geological Survey (Decades X and XII, 1861 and 1866), is at 

 present understood as comprising not more than six well recog- 



