IN THE PARAGUAYAN LEPIDOSIREN, ETC. 369 



and a posterior pair. Jugal plates are also present in Phaneropleuron, but are said to 

 be absent in Ctenodus 1 . 



iv. The presence of circumorbital ossicles. — In Dipterus there is series of bony plates 

 encircling the orbit and covering the cheek, and also forming the upper border of the 

 hinder part of the mouth [Traquair, 37]. According to Fritsch [I. c.~\, circumorbital 

 ossicles are probably present in Ctenodus, but of their existence in Phaneropleuron I 

 have been unable to find any evidence. 



v. The squamosal bone. — This element is certainly present in Ctenodus, and has been 

 described and figured by both Miall [23] and Fritsch (I. c.) as being very similar to its 

 homologue in Ceratodus. As to the character or even the presence of this bone in 

 the Dipteridge and Phaneropleuridse I can find no evidence. 



vi. The parasphenoid. — The parasphenoid of Dipterus and Ctenodus is known, but 

 not, so far as I am aware, in Phaneropleuron. It has been described and figured in 

 the first-mentioned genus by Traquair [37], and in Ctenodus by Barkas [1], Miall [23], 

 and Fritsch (I. c). In both Dipterus and Ctenodus the bone extends as far forward 

 as the palatine symphysis, thus filling up the whole space bounded anteriorly and 

 laterally by the palato-pterygoid bones, and in this respect these genera resemble 

 Ceratodus, while differing from Protopterus and Lepidosiren. In Dipterus the para- 

 sphenoid seems not to have extended beyond the posterior limit of the skull, but 

 in Ctenodus, as Barkas (I. c. p. 51) pointed out, the exceptional length of the bone 

 suggests that it extended for some distance beneath the vertebral column, thus 

 affording an additional point of agreement to the many which are apparent when the 

 skulls of Ctenodus and Ceratodus are compared. 



vii. The opercular bones. — There is perhaps some variation in the relative size of the 

 opercular bones in different extinct Dipnoi, but it is certain that they are usually 

 much larger than in any existing members of the group. 



vm. m The presence of ganoin. — As an investment to more or fewer of the skull-bones, 

 the presence of ganoin is most evident on the outer surface of the cranial roofing-bones, 

 the opercular bones, and the lower jaw of the Dipteridse. In Ctenodus and Phanero- 

 pleuron this characteristic investment is either entirely absent or restricted to fewer 

 bones, at all events so far as the cranial elements are concerned. 



As to the question whether the fossil Dipnoi are to be regarded as more primitive 

 than the living Dipnoi there is considerable difference of opinion. According to 

 Smith Woodward [45], the former are more specialized than any existing Dipnoi, 

 and the more generalized types have alone survived to represent the group at 

 the present day. In an elaborate and suggestive essay ' Sur la Phylogenie des 

 Dipneustes,' Dollo [7] has arrived at exactly opposite conclusions. According to his 

 views, Dipterus (D. valenciennesi) is to be considered the most primitive Dipnoid, the 



1 Traquair f39, P- 265] has expressed a doubt as to the assumed abseuce of jugal plates in this genus. 



