336 Mr. C. T. Eegan on the 



chiostegals, the anterior pair of which are considerably larger 

 than the rest and may be termed " gular plates." In front of 

 the gular plates there is sometimes an unpaired " intergular." 

 The anterior branchiostegals and the gular plates occupy 

 the whole of the space between the mandibular rami, to which 

 they are apposed, whilst each meets its fellow in the middle 

 line. Within the order Chondrostei the gular plates and 

 branchiostegals may disappear, but we never get the con- 

 ditions characteristic of either Crossopterygii or Teleostei. 



We have only just begun to realize that the clavicles 

 proper (infraclavicles) which Parker thought he recognized 

 in so many Teleostean fishes (Siluridje, Hemibranchii, Lopho- 

 branchii, Ostracion) are entirely wanting in that group, and 

 the presence of this bone as a distinct element in the Chon- 

 drostei and Crossopterygii becomes therefore of ordinal value. 



The arrangement of the bones of the cranial roof in the 

 Chondrostean Palceoniscidge is essentially similar to that of 

 the more generalized representatives of the other orders 

 (the Dipneusti excepted). Assuming the interfrontal pineal 

 foramen to be a primitive structure, we may expect to discover 

 a Paleeoniscid-like fish possessing this feature, and had such 

 a one existed in the early Silurian it would have been in 

 every way fitted to become the progenitor of the Teleostomi. 



Crossopteryqii. 



The Crossopterygii are modified Chondrostei, from which 

 order the more generalized forms differ but slightly. The 

 lobate pectoral fin has been shown by Dollo * to be an adaptive 

 s])ecialization, and is not to be regarded as of greater import- 

 ance than tlie lobate pectoral of some Teleosts (e. g. Ferio- 

 phthahnus, Pediculati) ; it may easily have been derived from 

 the Chondrostean type in the following manner: — 



The pectoral fin began to be used at times as a support for 

 the body, and even as an ambulatory limb. This change of 

 function produced a changed orientation in the muscular lobe 

 at the base of the fin, ■\vliicii, originally parallel to the body- 

 A\all and attached to it for its whole length, became set at an 

 angle to the body and detached from it posteriorly. As the 

 lobe separated tlie dermal rays extended round on to its inner 

 side, t The arrangement of the skeletal supports scarcely 



* Bull. Soc. Belg. Geol. ix. 1895, p. 79. 



t I am by no means satisfied that the pectoral fin of the extinct 

 genera Tristichopterus and EtistJmiopferon is correctly described as uni- 

 basal. That of 7'ristic/toptenis, as originally described and figured by 



