PISCES. 
91 
Bavaria and France, others of Ophiopsis and Histionotus from 
the Pur beck Stone of Dorsetshire and Wiltshire. 
The Pycnodontidae (“ thick-toothed ”) are a remarkable Wall-case 
family of deep-bodied fishes, so-called in allusion to the H- 
powerful grinding teeth (Fig. 97) which arm their forwardly- a 18 ® ^ 9 803 
displaced mouth. The rhombic scales are usually so thin 
that their ribbed front margin is often the only part pre¬ 
served, producing the appearance of a series of parallel 
streaks from the upper to the lower margin of the trunk. In 
Fig. 97.—Parts of the skeleton of Pycnodont Fishes, a. transverse section 
of jaws, showing the two halves of the mandibular dentition opposing 
the vomerine teeth ; b. vomerine and mandibular teeth of Microdon ; 
c. vomerine and mandibular teeth of Ccdodus; d. portion of vertebral 
column of Ccelodus , showing persistent notochord (shaded) and the 
expanded bases of the arches', e. the same of Pycnodus ; f. inner view 
of scales, showing mode of interlocking by pegs and socket s, which are 
continued as longitudinal ribs. (After J. J. Heckel.) 
several genera ( e.g ., Mesodon, Microdon , and Ccelodus ) the tail 
is destitute of scales. These fishes range from the Lower 
Lias ( Mesodon liassicus) to the Upper Eocene ( Pycnodus 
platessus) with very little modification. The fine series of 
examples of Gyrodus from the Lithographic Stone of Bavaria, 
and of Palceobalistum from the Hard Chalk of Mount 
Lebanon, are particularly worthy of attention. The armoured 
Coccodus and Xenopholis from Mount Lebanon are also 
remarkable. Hone of these fishes have vertebrae, but in the 
later genera the arches above and below the notochord are 
often expanded to unite at the sides (Fig. 97, e). 
