186 PEOF. W. K. PAEKBE ON THE STRTTCTUEE AND 



the main part (PI. XXXIX. fig. 4, b.hr^) is half the width of the interspace of the 

 mandibular rami, and is inversely heart-shaped, with a notched narrow as well as a 

 broad end. 



The hinder notch on the broad end is filled in with a small thick block of cartilage, 

 the rudimentary second basibranchial {b.br^) ; it has a convex fore margin ; but its sides 

 and end are concave ; with its sides the narrow rounded ends of the first ceratohyals 

 articulate. 



There is behind this part no cartilage along the mid line until we come to the larynx 

 {Ix). I am satisfied, however, from the study of other species that a basibranchial has 

 been absorbed, all but its fore part and its posterior horns (see in the larval Triton, 

 PI. XL. fig. 5, hM"-). The two small rods (PI. XXXIX. fig. 4, t.lnj) are apparently 

 the thyrohyals, or modified remnants of the last retral basibranchial. 



In Siren this retral part is retained (PL XXXIX. fig. 3, h.hr'-) ; and so it is in Proteus 

 and Menobranchus (Phil. Trans. 1877, pi. 28. fig. 1, b.br'-, and Proc. Zool. Soc. 1874, 

 March 17, pi. xxix. fig. 1, and pi. xxx. fig. 2, B.b'-). 



D. Sxhmmary of Characters of the Menopome's Skull. 



1. The breadth of the skull and the roundness of its general outline, except behind, 

 is noticeable ; this is combined with great flatness. 



2. The subpedunculate, large occipital condyles, with the intermediate scooped part 

 for the " odontoid rudiment." 



3. The large and perfect tract of cartilage between the exoccipitals and prootics. 



4. The large size of the nasal roofs and the confluent ethmo-palatines, and the arrest 

 of the axis between the nasal capsules, so that there is no projecting prenasal, the nasal 

 capsules projecting beyond the septum. 



5. T'his skull agrees with those oi Menohranchus, Siren, and Cryptobmnchus in having 

 all the three heads of the suspensorium confluent with the skull. 



6. The ethmo-palatine agrees with that of the more metamorphosed types in being 

 confluent with both the trabecula and the nasal capsule. 



7. The palatine bone, after giving ofi' the pterygoid, coalesces with the vomer. 



8. The pterygoid bone, after becoming free, grows to a great size, and ossifies most 

 of the largely developed pterygoid cartilage. 



9. There is a normal osseous centre in the quadrate region. 



10. The splenial is very large, almost as long as the ramus. 



11. The hyoid arch gives up its uppermost (" pharyngohyal ") segment to the audi- 

 toiy apparatus to form a columella by union with the stapes. 



12. The epihyal is confluent to some degree with the back of the suspensorium, and 

 is not a functional extrastapedial. 



13. The ceratohyal is half ossified above; there are three hypohyals and a small 

 retral basihyal. 



