OCR Text |
Show 1902.] PLECTOGXATHOUS FISHES. 289 short rough spine at the end of the pelvis. Most of the prae-caudal interneurals co-ossified to form a bony trough, attached to the skull, and receiving the retracted dorsal spines. In other characters similar to the two preceding families. Genera. 1. B alistes Linn.'-Body compressed, caudal peduncle short. Scales moderate or large, juxtaposed, osseous. Jaws usually even in front. Gill-openings behind the eyes. Dorsal usually with 3 spines-the first strong and just behind the eye, the second locking it when erected, the third, if present, remote from them. Soft dorsal with 23-35 rays. Anal with 20-30. Caudal rounded or truncate, the outer rays often more or less produced. Pelvis projecting. Ventrals represented by a short, rough, movable spine. The movable pelvis, abdominal sac, and ventral flap are very similar to those of Triodon, but much less developed. Palatine T-shaped, the cross-piece articulating with ethmoid and maxillary, the vertical limb with the ectopterygoid. Vertebrae 18. 2. M onacanthus C u v .-Differs from Balistes in that the scales are smaller ; the palatine is a straight rod attached to maxillary and ethmoid, having lost the lower vertical limb which in Balistes articulates with the ectopterygoid; the third dorsal spine is always absent and the caudal always rounded. The first dorsal spine is above the eye, and if it has distinct barbs these are usually arranged in two series. The gill-openings are often below the posterior part of the eye. There are 18 vertebrae. In this genus are included all those species of Monacanthus, as understood by Gunther, with a movable ventral spine. The transition is perfect from those with a rough dorsal spine without distinct barbs, to those with a series of minute barbs on each side, and so to those with barbs strongly developed. M. penicilligerus Cuv. belongs to this genus, the ventral spine being movable, and the barbs on the dorsal spine exactly similar to those of the closely-allied M. tomentosus, although their regular arrangement is obscured by the well-developed fleshy filaments. In the development of the ventral sac and flap, some species of this genus almost rival Triodon. 3. P araluteres Bleeker.- Differs from Monacanthus in that the single dorsal spine is weak, not fully erectile, and the ventral spine is absent, or small and fixed. 4. P seudaluteres Bleeker.- Differs from Monacanthus in that the dorsal spine is in advance of the orbit, the ventral spine is absent, and the pelvis is entirely concealed. 5. P seudomonacanthus Bleeker.- Differs from Monacanthus in that the ventral spine is immovable, ankylosed to the pelvis, and 1 The Oligocene genus Acanthodcrma Agassiz is scarcely distinguishable from Balistes. P r o c . Z o o l . S o c .-1902, V o l . II. No. X IX . 19 |