50 PISCES CLAss I 



Sub-Class 2. OSTRACODERMI. Cope. 



Early Palaeozoic fishes with persistent notochord, and endoskeleton not calcified. 

 Mandihle and arches for paired fiiis apparently ahsent. Exoskeleton well developed, 

 the head and anterior portion of the trunk usually covered with plates. Mouth 

 without hard parts. 



The Ostracoderras form one of the most characteristic groups of the early 

 Palaeozoic fish-fauna ; but their true systematic positioii is still uncertain. 

 Their remains were formerly compared with the armour of reptiles such as 

 tortoises, and also with crabs and water-beetles ; while more lately they have 

 even been supposed to exhibit resemblances to Arachnids. They were first 

 recognised as fishes by Agassiz, and referred by him to the Ganoidei. Unless 

 appearances in the fossils are deceptive, they are distinguished from all typical 

 fishes, and agree with the Cyclostomi in the lack of a lower jaw and of arches 

 for paired limbs ; but there seems to be evidence of paired nasal openings in 

 Pterichthys and Bothriolepis. 



Four Orders of Ostracophores may be recognised as follows : — Heterostraä, 

 Anaspida, Aspidocephali, and Antiarcha. In the first, the exoskeleton sometimes 

 consists of placoid tubercles of dentine, as in sharks ; in the second the 

 tubercles on the head region are never fused. In the more typical Hetero- 

 straci, as also in the Aspidocephali and Antiarcha, this primitive armour is 

 modified so that the head and anterior abdominal region become invested with 

 dermal plates, which are usually as well developed as in modern sturgeons 

 and certain bony fishes (Siluridae). The sub-class is restricted to the Upper 

 Silurian and Devonian. 



Order 1. HETEROSTRACI. Ray Lankester.i 



Exoskeleton consisting of shagreen, plates, or scales, without hone cells ; each fully 

 formed plate comprising three superimposed layers — an inner ^'nacreous" layer of 

 lamellae, a relatively thick middle zone with numerous vacuities, and an outer hard 

 layer of vaso-dentine. Dorsal shield, when present, of few pieces, and orhits wide 

 apart, laterally placed. No paired appendages. 



Family 1. Ooelolepidae. Pander. 



Head and trunk relatively large and depressed, completely covered with placoid 

 tubercles of dentine, scarcely if at all fused into groups ; the small, slender caudal 

 region sharply constricted from, the head r'egion and distinctly heterocercal. Upper 

 Silurian and Lower Devonian. 



The dermal tubercles of genera of this family have long been known from 

 the Upper Silurian of England and the Isle of Oesel in the Baltic Sea ; but 

 until the discovery of complete specimens in the Upper Silurian of Lanark- 



^ Alth, A. von, Ueber Pteraspis, Gyathaspis, iiiid Scaphaspis (Beitr. Palaeout. Oesteri.- 

 Ungarns, vol. II.), 188Q.—Huxley, T. IT., On üeplmlcisptis and Pteraspis (Quart. Journ. Geol. Soc. 

 voLs. XII., XIV., XVII.), 1856, 1858, 1%Q1.—Kner, R., Ueber Cephalaspis lloydü und levoisü 

 (Haidinger's Naturw. Abhandl. vol. I.), 1847. — Kunth, A., Ueber Pteraspis (Zeitschr. deutsch, 

 geol. Ges. vol. XXIV.), 1872. — Lankester, Ray, and Povyrie, J., A Monograph of the Fishes of the 

 Old Red Sandstone. I. Cephalaspidae (Mon. Palaeont. Soc), 1868. — lioemer, Ferd., Ueber Palaeo- 

 teuthis diinensis (Palaeontogr. vol. IV.), 1856. — Traquair, R. H., Report on Fossil Fishes, etc. 

 (Trans. Roy. Soc. Edinb. vol. XXXIX. Nos. 21, 32), 1899. 



