2 PISCES CLASS I 



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 Dicentroclus, Traquair ; Phoebodus, Lambdodus, Hyhodadodus, St. 

 John and Worthen ; Dicrenodits, Romanowsky (Carcharojjsis, Ag. ; Pristicladodus, 

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

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

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

 a possible Pleuropterygian. 



Order 2. ACANTHODII. Agassiz.i 



EndosJceleton well calcified, with dermal and membrane calcifications in the region 

 of the skull and pectoral arch. Pterygoqiiadrate arcade movabhj articulated with the 

 cranimn. Orbit surrounded tvith a ring of thin plates of dentine. Teeth, when 

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

 mamlibular cartilages. Endoskeletcd cartilages of cdl 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. Pelvic fins of male 

 toithout claspers. Dermcd armature of trunk consisting of smcdl, closely arranged, 

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

 passing hettoeen 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 Liitken, Fritsch, and Huxley. While, however, 

 they were arranged by Liitkeu 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 nvimber 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 

 {Haplacanthiis, Ag. ; Homacardhus, Ag. ; Gyracanthus, Ag. ; and Machaeracanthus, 

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

 Ichthvodorulites named Onchns suggests that at least some of the latter, which 

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

 Acanthodians. 



Family 1. Acanthodidae. Smith Woodward. 



A single dorsal fin. Free spines hettoeen paired fins usually absent, sometimes 

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



1 Fritsch, A., Fauna der Gaskolile in BohnK-n, vol. II. Prague. lSS9.—J/it.rle>/, T. II., Prelimi- 

 nary Essay upon the Systematic Arrangement of the Fishes of the Devonian Ej)och (Mem. Geol. 

 Snrv. dec. x. ), 1861. — Kner, It., Sitzungsb. k. Akad. Wiss. Wien, Math.-naturw. CI. vol. LVII. p. 

 290, 1S6S. — I'inrrie, ./., On the earliest known Vestiges of Vertebrate Life (Trans. Edinb. Geol. Soc. 

 vol. IX.), 1869. — Roenier, F., Vehnv Acuntltudis ijrarilis (Zuiiathr. deutsch. geol. Ges. vol. IX.), 1857. 

 — Reif, 0., Zur Kenutniss des Skelets der Acauthodinen (Geogn. Jahreshel'te, Miinchen), 1890 and 

 189 L-Traijmiir, li. If., Geol. Mag. 1888. p. 511, and 1889, p. 17. 



