STRUCTURES OF TELEOST 143 
laginous brain case is replaced by many definite osseous 
elements. The floor and roof of the skull, the face region, 
jaws, gill arches, and their protecting parts, are all encased 
by an elaborate series of membrane bones ; these, however, 
must be noted as deeply embedded in the body tissue, 
DCR, DN, A, 0, PT, SM, BR, O. The membrane bones 
of the jaw rim—maxillary, premaxillary, and dentary, JZX, 
PMX, DN— bear teeth, and are especially characteristic 
of the Teleostomes; those overlapping and protecting the 
gill arches (GA), O, JO, PO, SO, usually four in number, 
are also characteristic of the group. The skull is hyostylic. 
As to the visceral parts. The gill arches, GA, are 
reduced in number, usually widely bent backward, and 
closely crowded together ; their gill filaments are enlarged 
and specialized. The heart lacks the arterial cone with its 
transverse series of valves; in its place a stout bulbus, 2, 
forms the base of the aorta. The digestive tract is tubular, 
long, and coiled ; its intestine, G, lacks a spiral valve, and 
terminates at the body surface, AJ, not in a cloaca; its 
glands include a series, often great in number, of pyloric 
ezca (pancreas). An air-bladder, AB, is present, which 
may, or may not, retain its communication with the gullet. 
The ovary, with its many small eggs, and the kidney, dorsal 
to it, have often a common external opening in a urino- 
genital papilla, UG, in either side of which abdominal pores 
mayoccur. The nervous system and sense organs (pp. 275, 
277) have many peculiarities: the roof of the fore brain is 
 non-nervous ; the nasal openings appear in the dorsal side 
_ of the head, VO, and are separate; the eye has specialized a 
vascular, nutritive structure, the processus falciformis, pro- 
jecting from the region of the entrance of the optic nerve 
_ into the vitreous cavity of the capsule; the optic nerves 
__ Cross in passing to the eyes, but their fibres do not fuse. 
