hm.i 585 [Cope . 



notochordal sturgeons as descendants of Crossopterygia, whose modern 

 representatives are osseous. The primitive Crossopterygia, and probably 

 even the Actinopteri, were doubtless as cartilaginous as are the existing 

 sturgeons : 



r Actinopteri. 

 Ilyopomata = 1 Chondrostei. Batrachia. 



( Crossopterygia. / 



IchthyotomL Dipnoi. 



Selachii. / 



-~-\JrIolocephali. / 



Elasmobranchi s=. 



In this phylogeny, the Holocephali, which have not differentiated a 

 suspensorium, are regarded as the primitive fishes; although the living 

 representatives display eome specialized character?, as, for instance, a 

 membranous gill-cover which conceals the primitive slits. The line to 

 the right continues the monimostylic character and passes into the reptiles, 

 whose primitive types are also monimostylic, as Johannes Miiller called 

 them. In the later forms or streptostylicate reptiles of Miiller (Lacertilia, 

 Ophidia), the quadrate becomes freely articulated.* 



In the left hand series, the Elasmobranchs immediately present us with 

 the free suspensorium or hyomandilmlar, which is a well-known character 

 of the remainder of the line, the modifications being the addition of sepa- 

 rate elements, as the metapterygoid, "quadrate," and symplectic. 



The penetration of ossification into the chondrocranium of Didymodus, 

 in regions not ossified in either fishes or batrachia (sphenoid and pre- 

 sphenoid), and into regions not ossified in any vertebrate (frontal and 

 parietal cartilages), may be, so to speak, only a local phenomenon, and 

 not indicative of extensive phylogenetic consequences. For if it be so 

 regarded,, it evidently proves too much, giving affinities in the base of the 

 skull to the reptiles, and in the roof exhibiting a character more highly 

 developed than any known form of vertebrata. 



The Ichthyotomi include, so far as yet known, but one family, the Hybo- 

 dontidse of Agassiz. According to that author this family includes four 

 genera, Hybodus, Pleuracanthus, Cladodus and Sphenonclius. It ranges 

 from the coal-measures to the Jura inclusive. 



The genus Didymodus may be described as follows : 



Frontal plane well defined on each side by the temporal fossae, and ter- 

 minating in two cornua posteriorly. Anterior nares on the superior sur- 

 face of the muzzle. Supraorbital (or nasal) bones well separated on the 

 median line and constituting the only membrane ossification Teeth with 

 large lateral denticles. 



The species Didymodus compressus N'ewberry, may be defined as follows : 



Skull with massive walls. Form elongate, depressed, the orbit not ex- 



* The phytogeny of the Reptilian series can be found in the Proceedings 

 American Association Advancement of Science, xix, 1871, p. 233. The Batrachia 

 are supposed to be their ancestors. 



