IN THE PAEAGUAYAX LEPIDOSIREN, ETC. 327 



The skull of the remaining Dipnoid, Ceratodus, was first described and figured by 

 Giinther [i i ], whose account was subsequently revised, and also amplified in certain 

 particulars, in an eminently suggestive paper by Huxley [15]. 



The object of the present communication is {a) to place in the hands of morpho- 

 logists the results of a detailed investigation into the structure of the skull of the 

 Paraguayan Lepidosiren, illustrated by accurate and carefully-drawn figures; (b) to 

 revise the accounts of the skulls of Protopterus and Ceratodus given by preceding 

 writers ; and (c) to institute a more detailed comparison of the differences and 

 resemblances between the various Dipnoid skulls than has yet been made. 



I desire also to express my thanks to the Council of the Royal Society for a grant 

 from the Research Fund in aid of this and other investigations. 



II. Description of the Skull of the Paraguayan Lepidosiren. 



In somewhat striking contrast to the relatively short and bluntly-conical shape of 

 the head, the skull, when stripped of its investing skin and powerful muscles, appears 

 relatively longer and more sharply conical in contour. The narrowest portion of the 

 skull is the central region, or that part which lies directly behind the eyes, expanding 

 anteriorly in the nasal region, and widening even more behind in the auditory and 

 laterally-deflected suspensorial regions. Viewed laterally the skull presents a strikingly 

 carnivorous appearance, largely clue to the existence of a prominent sagittal crest along 

 the medio-dorsal line of the fronto-parietal bone, and to the backwardly-projecting 

 " lambdoid " margin in which that bone terminates dorsad to the occipital plane. 



Of the various cranial bones the fronto-parietal (the parieto-frontal of Bischoff and 

 Hyrtl ; in Protopterus, the parietal of Owen, Peters, and Cobbold, and the " Fronto- 

 parietal " of Wiedersheim) (PL XXVIII. figs. 1, 2, and 4 ; PI. XXIX. figs. 13-19/p.), 

 from the share which it takes in the formation of the roof and side-walls of the cranial 

 cavity, is perhaps the most important. In shape each lateral half of the bone is 

 somewhat triangular (fig. 1), the broad bases of the two halves meeting in the medio- 

 dorsal line, and there forming a strong longitudinal sagittal crest or ridge (figs. 1, 2, 

 and 4, sg.c.) which serves for the origin of the more superficial portion of the temporal 

 muscle. In no other Fish with which I am acquainted are the temporal muscles so 

 powerfully developed in proportion to the size of the skull as in Lepidosiren, and in 

 none do these muscles extend so far on to the dorsal surface of the skull. It is to 

 the exceptional development of the temporal and masseter muscles that the bluntly- 

 conical shape of the head is mainly due, since they are principally responsible for the 

 thickening of what, so far as the skull alone is concerned, would otherwise be almost 

 the narrowest portion of the head. From the sagittal crest the lateral portions of 

 the fronto-parietal extend downward and outward, and form a gable-roof for all that 



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