IN THE PARAGUAYAN LEPIDOSIREN, ETC. 337 



side immediately above the upper extremity of the squamosal. In a ventral view, 

 however, each capsule is well seen between the foramina for the Facialis and Vagus 

 nerves, the rounded prominence which it presents in this region corresponding to the 

 outer wall of the recess for the sacculus (fig. 3) 



In a vertical longitudinal section of the skull (PI. XXVIII. figs. 4 and 6) the cavity 

 of the periotic capsule appears as a relatively spacious lateral diverticulum of the 

 cranial cavity, the inner wall of the capsule, which in Elasmobranchs and Amphibians 

 separates the two cavities, having completely atrophied in Lepidosiren, as in Ceratodus 

 and Protopferus, and also in all existing Ganoids and Teleosts. The cavity of the 

 capsule exhibits a division into two deep recesses, of which the more external and 

 dorsal lodges the "pars superior" (utriculus) (fig. 6, ut.r.) of the auditory organ, 

 while the more internal and inferior contains the " pars inferior " or sacculus. The 

 numerous foramina perforating the periotic capsule will be referred to subsequently. 



The suspensorium consists of a somewhat triangular lamina of cartilage, the broad 

 base of which is continuous dorsally with the outer wall of the auditory capsule and 

 with the hinder portion of the trabecular cartilage, and thence is continued obliquely 

 downward and forward to the articular condyle for the lower jaw (PL XXVIII. 

 fig. 3, and PL XXIX. fig. 18, ar.c). So complete is the investment of its external 

 surface by the fronto-parietal and squamosal bones that only a small portion of the 

 cartilage is visible just above the mandibular articulation (PI. XXVIII. fig. 1). 

 Ventrally, however, the suspensorial cartilage (PL XXVIII. fig. 3) may be seen 

 anteriorly and externally to the foramen for the exit of the hyomandibular branch of 

 the Facial nerve, but even here the cartilage is extensively invested by the hinder part 

 of the palato-pterygoid. The posterior margin of the suspensorial cartilage is deeply 

 emarginate (fig. 3), so that in a ventral view the inner surface of the squamosal is 

 partially exposed, and in the same view a rounded prominence near the hinder margin 

 of the cartilage indicates the point of attachment of the hyoid arch. A charac- 

 teristic foramen termed by Hyrtl (I. c.) the " Schlafengrube " perforates the suspensorial 

 cartilage at its junction with the periotic capsule (PL XXVIII. figs. 1 and 2, t.f. ; 

 PL XXIX. fig. 18), and in a ventral view is visible near the base of the skull (fig. 3). 



The suspensorial cartilage of Lepidosiren must be regarded as the metapterygo- 

 quadrate or proximal portion of the mandibular arch, the equivalent of the palato- 

 pterygoid cartilage of the more typically autostylic skulls of Chimmra and Ceratodus, 

 having undergone complete atrophy or become replaced by the palato-pterygoid bone. 

 It is worthy of note that the suspensorium makes a much more open angle with the 

 fore part of the basicranial axis than in Protopterus or in Ceratodus, and the effect of 

 this on the curvature of the palato-pterygoid bone is such that, in passing from the 

 articular end of the suspensorium to the nasal region, the bone describes a segment of 

 a larger circle in the former Dipnoid than in either of its congeners. 



The pulley-like articular condyle for the mandible (PL XXVIII. figs. 1, 2, 3 and 4, 



