TELEOSTOML 549 



Chimseroids, occur in Devonian rocks, and some at least of the 

 detached spines of Carboniferous age may have belonged to fishes 

 of this order. Undoubted Mesozoic Chimreroids are Squaloraja, 

 Myriacanthus, Chinueropsis, Ischyodus, etc., while others, includ- 

 ing the recent genus Chinufra, are found in strata of Tertiary age. 

 The other recent genus, Ca/lorhynchns, is also represented by a 

 Cretaceous species, C. hectori. 



EXTINCT ORDERS. 

 Order 3. PLEUROPTERYGII. 



Devonian, Carboniferous, and Permian. Forms with unconstricted 

 notochord, heterocercal tail, terminal mouth, paired fins with unseg- 

 mented parallel radials. Cladoselache. 



Order 4. ICHTHYOTOMI. 



Lower Carboniferous to Permian. Forms with unconstricted 

 notochord, diphycercal tail, and pectoral fins with a segmented 

 axis of basals bearing biserial radials. Pleuracanthns. 



Order 5. AcANTHODEi. (See Fig. 266. ) 



Another interesting extinct group, whose position was for long a 

 matter of dispute, but which is now usually placed near Elasmobranchii, 

 is that of the Acanthodei. These flourished principally in Devonian 

 times, but lived on through the Carboniferous to the Lower Permian. 

 They are usually rather small fishes, with minute rhomboidal shagreen- 

 like scales, and a strong spine in front of each fin, except the caudal. 

 In some genera (Parexiis, Cli matins} there are two rows of small 

 intermediate spines between the proper pectorals and the ventrals. 



Sub-Class II. TELEOSTOMI. 



Fishes with more or less ossified skeletons, especially as 

 regards skull, jaws, operculum, and pectoral girdle. The 

 skull is hyostylic, i.e. supported by the hyomandibular. 

 The pelvic girdles are usually rudimentary or absent. The 

 mouth is usually terminal ; the scales are in the majority 

 soft and cycloid. There is always a gill-cover ; the inter- 

 branchial septa are much reduced ; the gill-filaments pro- 

 ject freely from the gill-arches. There is usually a swim- 

 bladder. There are no claspers, no naso-buccal grooves ; 

 there is no cloaca. The fore-brain has a non-nervous roof. 

 The ova are small and numerous, usually meroblastic, 

 sometimes holoblastic. Fertilisation is usually external. 



