IN THE PARAGUAYAN LEPIDOSIEEN, ETC. 333 



it becomes greatly thickened by the addition of fibres having their origin from the outer 

 surface of the fronto-parietal bone, including its sagittal crest ; from the median vertical 

 fibrous sheet which continues the crest as far externally as the superficial temporal fascia, 

 and alone in this region separates the temporal muscles of opposite sides of the head ; and 

 also from the outer margin and ventral surface of the hinder two-thirds, or postorbital 

 portion, of the external ectethmoid. From this extensive origin the Temporalis runs 

 obliquely downward and forward, anteriorly and externally to the much less 

 developed Masseter muscle, to its insertion into the coronoid process of the mandible. 

 These facts suggest that the unusual backward extension of the external ectethmoid 

 is due to the necessity of providing an additional surface for the origin of an excep- 

 tionally developed muscle. It is not only possible, but even probable, that the hinder 

 or postorbital section of each ectethmoid owes its existence to the ossification of the 

 temporal fascia investing this muscle, and to this extent it is possible to agree with 

 Wiedersheim that the bone is a " Sehnenbein." Nevertheless, and for the reasons 

 mentioned above, it may be maintained that the orbital and preorbital portions of the 

 bone represent a true " external ectethmoid " element. 



The identification of the " supraorbital " bones with the Amphibian " external 

 ectethmoids" removes almost the only difficulty in the way of the comparison of the 

 cranial bones of the Dipnoi with those of other Vertebrata. Parker's term " external 

 ectethmoid " is not, perhaps, the most suitable name for the bones under discussion ; 

 dermal ectethmoids, or, in order to distinguish these paired bones from the median 

 dermal ethmoid and the true mesethmoid bone, dermal lateral ethmoids, are much 

 more convenient terms. 



Underlying the base of the cranium, and extending backward from a point at some 

 distance behind the symphysis of the palato-pterygoid bones to the level of the first 

 neural arch, is the well-developed parasphenoid (" Keilbein " of Bischoff'; in Protopterus 

 the " Basilare " of Peters, " sphenoid " of Cobbold, " basioccipito-sphenoid " of Owen, 

 and the " parasphenoid " of Huxley and Wiedersheim ; in Ceratodus the " basal " bone 

 of Giinther, and " parasphenoid " of Huxley), (PI. XXVIII. figs. 3 and 4 ; PI. XXIX. 

 figs. 16-19, jps.). Viewed from below (fig. 3) the bone appears somewliat spatulate 

 in shape, the anterior half widening considerably, aud having its lateral margins 

 deflected and at the same time closely applied to the inner surfaces of the palato- 

 pterygoid bones, more particularly where the latter bones curve downward to form the 

 articular condyles for the lower jaw. Anteriorly, the bone terminates at some distance 

 behind the palato-pterygoid symphysis, not, however, as in Protopterus, by an abruptly- 

 truncated transverse margin, but by a contracted and somewhat conical extremity. 

 From the auditory region backward the bone gradually narrows, and by lateral 

 compression becomes greatly thickened, eventually terminating in an almost pointed 

 extremity ventrad to the basal cartilage of the occipital legion. The share taken by 

 the parasphenoid in conjunction with the fronto-parietal in forming the side-walls 

 VOL. XIV.— PART v. No. 2. — February, 1898, 2 x 



