1849.] EGERTON ON THE GENUS PLATYSOMUS. 331 



not restricted to the dorsal region of the fish, but extend uninter- 

 ruptedly from the back to the belly, external to the vertebral column 

 and apophyses. Secondly, they are continuous with, not articulated 

 to, the external spines or scales before the dorsal fin. The dermal 

 system of the Pycnodonts is very peculiar. Each scale bears upon 

 its inner anterior margin a thick solid bony rib, extending upwards 

 beyond the limits of the scale, and sliced off obliquely above and 

 below, on opposite sides, for forming splices with the corresponding 

 processes of the adjoining scales'*^. These splices are so closely ad- 

 justed, that without a magnifying power or an accidental dislocation 

 they are not perceptible. When in situ and seen internally these con- 

 tinuous lines decussate with the true vertebral apophyses, and cause 

 the regular lozenge-shaped pattern so characteristic of the Pycnodont 

 familyf. In all the species oi Platysornus these structural peculiari- 

 ties obtain to their fullest extent, corroborating the evidence already 

 alluded to of the masticatory organs, in favour of the removal of the 

 genus from the Heterocerque Lepidoids to the Pycnodonts. The 

 explanation I have ventured to advocate as to the true nature of the 

 enigmatical bones designated by Agassiz "apophyses," is shown to 

 be correct as to the genera Gyrodus and Microdo7i by several perfect 

 specimens of the former genus in my cabinet, which I owe to the 

 kindness of the late Count Miinster, and by a fine example of Micro- 

 don radiatus in the Hunterian collection. There is more difficulty 

 with the genus Pycnodus, for in this the scales are much thinner 

 than in the other members of the family, and the articulating 

 border more delicate ; the latter feature is nevertheless generally 

 preserved, even when the other portions of the scales are wanting, 

 and has great resemblance to a vertebral apophysis, more especially 

 when slightly crushed, for then the compound nature of the bone is 

 undistinguishable. Its real dermal character may, however, generally 

 be ascertained by examining the impressions of the inner surfaces of 

 the scales of the opposite flank. If it be true then that these bones 

 form part of the cuticular investment ai^fl not of the internal skeleton, 

 we have still the difficulty of explaining how it happens that this 

 peculiar structure is restricted in some species to the anterior region 

 of the body, while in others it extends to the insertion of the tail. 

 It cannot be attributed to any imperfection of the specimens, for the 

 scalpel of the most skilful zootomist could scarcely exhibit more 

 beautiful dissections than Nature has placed before us in the Pyc- 

 nodonts of Kelheim, Torre d' Orlando and Monte Bolca. The only 

 solution I can suggest is this, that whereas in some recent fishes we 

 find a stiff and rigid body furnished with a flexible tail, so a like 

 compensation may have obtained among some of the denizens of the 

 more ancient seas. Of the fossil Pycnodonts already known, the 

 Platysomi, Gyrodi, Wicrodon hexagonusX and Microdon anaUs% 

 have the scales uniform, while Microdon elegans\\ and Mid'odon 



* Poiss. Foss. tabs. 68, 69. figs. 2, 3. f Ibid. vol. ii. tab. 67. 



X Ibid. vol. ii. tab. 69<-. figs. 4 & 5. § Ibid. vol. ii. tab. 09'-. fig. 3. 



II Ibid. tab. 69b. 



VOL. V. PART I. 2 A 



