74 PISCES. 



two dorsal fins, both remote and the foremost nearly opposite the pelvic 

 pair ; and their supporting elements are arranged in the sub-genus Olypto- 

 lepis as shown in fig. 19, p. 22. The anal fin is separate, though small; 

 while the caudal fin is heterocercal, the upper lobe small, the lower lobe 

 triangular and obliquely truncated. The typical species, H. nobilissimus, 

 and another well-known form, H.flemingi (fig. 55), occur in the Upper Old 

 Eed Sandstone of South Scotland, perhaps also in the Upper Devonian of 

 Belgium and N.W. Russia. Other fragmentary remains are found in the 

 Upper Devonian of North America. Holoptychius (Glyptolepis) leptopterus 

 is abundant in the somewhat earlier Old Eed Sandstone of the north of 

 Scotland. 



The Rhizodontidae exhibit the teeth with a larger central 

 cavity and a less complicated infolding of its walls than is 

 observable in the previous family. In this respect they are 

 thus less specialized than the Holoptychiidae (Dendrodontidse) ; 

 but they often show robust ossifications in the notochordal 

 sheath, and the paired fins are always obtusely lobate, more or 

 less abbreviated. Tristichopterus, Eusthenopteron, and Gyro- 

 ptychius are well-known Devonian genera, while Rhizodopsis is 

 equally well preserved in the Carboniferous. Rhizodus and 

 Strepsodus are very large fishes known only by fragmentary 

 specimens from the Carboniferous. 



Rhizodus is confined to the Lower Carboniferous, and the 

 mandibular ramus of R. hibberti (fig. 56 A) from the Edinburgh 

 district usually measures about 0'3 m. in length. The largest 

 teeth are about 012 m. in height, and compressed to a pair of 

 sharp edges : they probably represent a jaw O6 m. in length. 

 The most remarkable feature of the genus is the extremely firm 

 union of the infraclavicle with the clavicle, the former element 

 (fig. 56 c) bearing a long upwardly-directed process apposed to 

 the hollow on the inner face of the latter (fig. 56 B), which is 

 slightly twisted at its middle. Strepsodus appears to be almost 

 identical, except that its teeth are round in section; the 

 species range throughout the Carboniferous, but the largest 

 and finest forms occur in the Coal Measures proper. 



Rhizodopsis (fig. 57). In this, as in the other Rhizodonts, the cranial 

 cartilage is 'in some degree ossified. It is entirely covered with thick 

 membrane-bones or dermal plates, exhibiting a bilaterally-symmetrical 

 arrangement except towards the end of the rostrum ; and there is, 



