250 DIPNOI. 



at the hind end of the tail and in the first two or three segments), 

 and intercalated pieces may occur. The ribs, arches, and fin- 

 supports show a tendency to ossification. 



The chondrocranium consists almost entirely of cartilage. In 

 Ceratodus it is massive and completely developed, there being 

 no fontanelles in the roof or floor. In Lepidosiren the trabeculae 

 cranii have retained their primitive condition of rods of cartilage 

 bounding a large pituitary fontanelle, and have not extended 

 dorsally, so that the greater part of the side-walls and roof are 

 formed of membrane bone. The notochord is continued into 

 the base of the skull, with which the notochordal sheath of the 

 vertebral column is continuously and immovably united. In 

 Lepidosiren and Protopterus the first distinguishable neural arch is 

 ossified and placed behind the foramen for the second spinal 

 nerve, and the first rib (cranial rib) is articulated to the occipital 

 cartilage. In Ceratodus it would appear that the number of neural 

 arches which have been incorporated in the skull is greater. 

 The two exoccipital bones are the only cartilage bones found in 

 the chondrocranium.* The space containing the membranous 

 labyrinth is open to the cranial cavity as in Ganoids, Holocephali, 

 and Teleosteans. The skull is completely covered dorsally 

 (except over a part of the ethmoid cartilage) by dermal mem- 

 brane bones, on the floor there is a parasphenoid and a trace of 

 two vomers in the two chisel-shaped vomerine teeth in the front 

 of the roof of the mouth. There are two large palatopterygoid 

 bones, wliich meet in front beneath the ethmoid and carry two 

 large, tuberculated, palatal teeth. Maxillae and premaxillae 

 are absent. The suspensorium has the form of a triangular 

 shelf, continuous with and projecting from the skull, and sup- 

 ported by the stout pteiygo-palatine bone in front and by the 

 squamosal behind. The quadrate region of this suspensorium 

 is unossified. Meckel's cartilage is persistent and ensheathed 

 by two, sometimes three, membrane bones, of which the splenials 

 are of unusual size and carry the two large tuberculated teeth. 

 The teeth consist of fenestrated bony tissue continuous with 

 that of the jaws, and of dentine covered by enamel on the exposed 

 tubercles. The hyoid is attached to the hinder upper part of 

 the suspensorium where it passes into the auditory cartilage, 



* Fiirbringer {Anat. Anzeiger, 24) finds that these do not belong to 

 the chondrocranium, but are the neural arch of an incorporated vertebra. 



