IN THE PARAGUAYAN LEPIDOSIREN, ETC. 339 



and finally terminates in the inferior margin of the labial fold (PI. XXVIII. figs. 1-4, 

 an.p'). Although regarded by both Bischoff (I. c.) and Hyrtl (I. c.) as pertaining to 

 the system of labial cartilages, there can, I think, be little doubt that, from the point 

 of its separation from the cartilaginous basis cranii, each of these singular rods 

 represents an "antorbital process," 1 almost identical in its mode of origin and 

 relations with a homologous cartilaginous process which exists in many Elasmobranchs 

 (the "seitlicher Vorsatz cler Ethmoidal-Eegion " of Gegenbaur, 10, or "antorbital 

 or ethmo-palatine process" of Parker, 29), and more especially in the Urodele 

 Amphibia ("antorbital process " of Wiedersheim, 43, or "ethmo-palatine cartilage" of 

 Parker, 28, 30). Ordinarily in both Elasmobranchs and Urodela the process is 

 either directed outward at right angles to the axis of the skull, or inclined slightly 

 backward, but it is interesting to note that in some Urodela (e. g. Menopoma and 

 Siredon) the antorbital process is directed forward as in Lepidosiren, parallel to the 

 outer margin of the nasal capsule, with which it may even fuse anteriorly (Menopoma), 

 or, as in Siredon, remain distinct (Wiedersheim, 43). 



The singular development of the antorbital process in Lepidosiren is evidently 

 associated with its function as a skeletal support for the posterior portion of the 

 overlapping and somewhat pendulous upper labial fold of this Dipnoid. It may 

 be mentioned that besides its skeletal support the labial fold is provided with 

 special muscles, either as derivatives from the anterior portion of the contiguous 

 temporal muscle, or arising independently from the temporal fascia (Hyrtl, I.e.), and 

 also with an abundant nerve-supply by the ramus buccalis of the Facialis nerve. 



After the separation and divergence of the two antorbital cartilages, the cartilaginous 

 basis cranii becomes pushed upward, as it were, by the palato-pterygoid symphysis, and. 

 rapidly contracting in width, fuses with the nasal roof in the median line, dorsad to the 

 symphysis and beneath the dermal ethmoid (fig. 4). Immediately posterior to the 

 palatine symphysis a vertical mesethmoid cartilage divides the much-contracted anterior 

 section of the cranial cavity into two short, laterally-situated, tubular passages for the 

 transmission of the Olfactory nerves to the nasal sacs (fig. 4, and PI. XXIX. fig. 13, 

 ms.e.). Dorsally and posteriorly the mesethmoid cartilage is prolonged into a median 

 styliform process (PL XXVIII. fig. 5, and PI. XXIX. fig. 14, st.p.) which extends 

 backward for a short distance in the cranial roof internally to the fronto-parietal bone, 

 and is presumably a remnant of the more extensive cartilaginous cranial roof of the 

 embryo. Inferiorly, the mesethmoid cartilage is coincident with a small oval vacuity 

 in the basicranial cartilage, through which it may be seen, in a ventral view of the skull, 

 immediately behind the palatine symphysis (PI. XXVIII. fig. 3, and PI. XXIX. fig. 13, 

 b.c.v.). Anteriorly to this vacuity the vertical extent of the mesethmoid becomes 

 rapidly diminished by the upward growth of the symphysis, and finally, after blending 



1 Wiedersheim (I. c.) was the first to regard the representatives of these cartilages in Protopterus from 

 this point of view. 



