MACROPOMA MANTELLII. 



31 



lous extremity has been closed up by an elastic membranous append- 

 age capable of acting as a valve, this would enable the creature to 

 regulate its buoyancy, by increasing or diminishing the compression of 

 the contained air, and thus facilitate its movements in either shallow 

 water or at great depths. Except in cases of diseased ossificatiou, the 

 existence of an internal thoracic or abdominal viscus having hard 

 parietes of true bone, is an anomaly which, as far as I am aware, has 

 hitherto presented no parallel in nature." 



Professor Williamson finally remarks that the structure of — 



" The scale of Macropoma, as now described, is wholly different 

 from that presented by any of the ganoid fish noticed in the preceding 

 pages [^Lepidosteus, Lepidotus, Semionotus, Pholidotus, Ptycholepis, 

 Dapedius, PalcEoniscus, Gyrodus, Aspido7'hynchus, Accipenser, Platy- 

 somus, Megalichthys, Diplopterus, Holoptychius*\ It bears a much 

 closer resemblance In its leading points to the dermal appendages found 

 amongst the group of true Placoids, between which and the Ganoids 

 Macropoma appears to form an inosculating link." (L.c. p. 464.) 



In 1857, Sir Philip Egerton described and figured the specimen 

 named Macropoma Egertoni by Agassiz, in the ninth of our 

 " Decades," PI. 10. This fossil, which was obtained from the 

 Gault of Speeton, Yorkshire, exhibits only the anterior half of 

 the fish, comprising the head, the pectoral fin, and the first few 

 rays of the dorsal fin. 



The inclination of the profile line of the head is very steep from 

 the occiput to the orbit, far more so than in the allied species ; 

 the orbit is situated in a more advanced position, and the facial line 

 thence to the snout falls much more rapidly. The orbit in this species 

 is large, and a portion of the capsule of the eye is preserved. The 

 frontal bones are wide, coarse in texture, and bear a few scattered 

 granules on their exterior surface. The borders of the upper jaw are 

 formed of the superior maxillary bones, which are very broad ; they are 

 beset with very numerous sharp-pointed teeth, closely arranged, and of 

 uniform size. The lower jaws are also very broad, and the space 

 between the rami is closed by a single glossohyal plate, as in Lophio- 

 stomus, Arapaima, and Amia." 



As regards the scales. Sir Philip Egerton remarks that — 



" The surface ornament is very different ; instead of the distinct 

 tubercles so characteristic of that species (M. Mantellii) ; it is composed 

 of minute granules united into longitudinal rows, with only a few small 

 tubercles interspersed occasionally on some of the larger scales." 



I venture to doubt that this fish belongs to the genus Macro- 

 poma, for the following reasons : 



10. The upper contour of the skull is quite unlike that which 

 is seen in Macropoma and appears to be characteristic of the Coela- 

 canths in general. 



20 In no Macropoma, and in no Coelacanth, however well pre- 

 served, have I seen any trace of an ossified capsule of the eye- 

 ball. 



* This is not the true Holoptychius of the Old Red, but a distinct genus, Mhizo- 

 dopsis. 



