20 PISCES CLASSi 



not all belong to the Pleuropterygii. Cladodus neilsoni, from the Calciferous 

 Sandstones of East Kilbride, Lanarkshire, is known by the imperfect head 

 associated with a very remarkable pair of pectoral fins (Traquair, Trans. Geol. 

 Soc, Glasgow, vol. xi. 1897, p. 41). 



The genera Dicentrodus, Traquair ; Fhoebodus, Lambdodus, Hyhodadodiis, St. 

 John and Worthen ; Dicrenodus, Komanowsky (Carcharopsis, Ag. ; Fristidadodus, 

 M'Coy), are founded on detached teeth, mostly from the Carboniferous Lime- 

 stone, of uncertain afiSnities. Protodus, Smith Woodward, founded on teeth 

 from the Lovver Devonian of Canada and Scotland, may also be mentioned as 

 a possible Pleuropterygian. 



Order 2. ACANTHODII. Agassiz.i 



Endoskeleton well calcified, with dermal and memhrane calcifications in the region 

 of the skull and pedoral arch. Pterygoquadrate arcade movahly articidated with the 

 cranium. Orbit surrounded with a ring of thin plates of dentine. Teeth, when 

 present, firmly fixed to the calcified sheathing plates of the pterygoquadrate and 

 mandibular cartilages. Endoskeletal cartilages of all the fins much reduced, and the 

 dermal expansion almost or completely destitute of rays ; each of the fins except the 

 caudal with a robust anterior spine implanted in the flesh. Felvic fins of male 

 without daspers. Dermal armature of trunk consisting of small, closely arranged, 

 quadrate granules, which also extend over the greater portion of the fins ; lateral line 

 passing between two series of the granules. 



Two families of this order are distinguished, the Acanthodidae with one 

 dorsal fin, and the Diplacanthidae with two dorsal fins. These fishes were 

 originally assigned to the Ganoidei by Agassiz, but their close relationships to 

 the Selachii were observed by Lütken, Fritsch, and Huxley. While, however, 

 they were arranged by Lütken and Fritsch among the Selachii, they were 

 regarded by Huxley as intermediate between the Ganoidei and Selachii. 

 Later researches by Traquair, Smith Woodward, and Reis seem to have 

 justified the reference of the order to the Selachii. 



The Acanthodii comprise not only a number of more or less completely 

 known genera from the Devonian, Carboniferous, and Permian formations, 

 but also a few genera which are still represented only by isolated fin spines 

 (Haplacanthus, Ag. ; Homacanthus, Ag. ; Gyracanthus, Ag. ; and Machaeracanthus, 

 Newberry). The close resemblance between the spines of Climatius and the 

 Ichthyodorulites named Onchus suggests that at least some of the latter, which 

 ränge throughout the Upper Silurian and Lower Devonian, also belong to 

 Acanthodians. 



Family 1. Acanthodidae. Smith Wood ward. 



A Single dorsal fin. Free spines between paired fins usually absenf, sometimes 

 'represented by one rudimentary pair. Lower Devonian to Lower Permian. 



^ Fritsch, A., Fauna der Gaskohle in Böhmen, vol. II. Prague, 1%%^.— Huxley, T. H., Prelimi- 

 nary Essay upon the Systeraatic Arrangement of the Fishes of the Devonian Epoch (Meni. Geol. 

 Snrv. dec. x.), \%&\.—Kner, R., Sitzungsb. k. Akad. Wiss. Wien, Math.-naturw. Gl. vol. LVII. p. 

 290, l%Q9>.—Poiorie, J., On the earliest known Vestiges of Vertebrate Life (Trans. Edinb. Geol. Soc. 

 vol. IX.), 1869.— ßoemg/-, F., lieber Acantlwdes gracilis (Zeitschr. deutsch, geol. Ges. vol. IX.), 1857. 

 —Reif, 0., Zur Kenntniss des Skelets der Aeanthodineu (Geogn. Jahreshefte, München), 1890 and 

 189L—Traquair, R. IL, Geol. Mag. 1888, p. 511, and 1889, p. 17. 



