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VII. SIRENIA. 
FAMILIES. 
I. Tail entire, rounded, and with the vertebrae towards last (i.e. 5+); 
sub-cylindrical and destitute of transverse processes, Intermaxillary 
bones with the branches little prolonged backwards and with the anterior 
portion nearly or quite straight. (Tvrichechoidea.) 
TRICHECHIDAE. (LX.) 
II. Tail forked, and with the vertebrae (except the terminal) depressed and 
provided with transverse processes. Intermaxillary bones (attypically) 
with the branches prolonged backwards and with the anterior portion more 
or less deflected. (Halicoroidea.) 
A. Teeth present, and in part at least functionally developed in the adult: 
molars 2 to §X2 in number, but rarely present in full complement, the 
anterior being gradually cast; incisors in the upper jaw two (more or less 
prominent) at least inthe male. Teeth of the complete series—at. least 
of Trichechidae—M 378, C 9,1 22; the upper incisors only persistent, 
the others as well as the canines being reabsorbed; molars successively 
increasing’ in size backwards. 
1. Molars mostly with two or three roots each (generally three-rooted 
above and two-rooted below) ; and with crowns furnished with obtuse 
tubercles arranged in transverse yoke-like eminences, and in the 
posterior ones with an additional narrower tuberculated yoke behind 
the principal ones. (Lrandt.) 
HALITHERIIDAE. (LXI.) 
2. Molars with simple hollow roots (not separated from the crowns) ; 
with crowns furnished with little prominent tubercles, few in number 
(in the anterior teeth simple or double, in the rest three or four) not 
forming yoke-like eminences, and early worn away and disappearing. 
HALICORIDAE. (LXII.) 
B. Teeth absent. (Intermaxillary lines with the apical portion produced 
and simulating incisorial teeth. Manducation’ is only effected by a 
very large palatine corneous plate, and by another opposed to it and 
covering the very large and elongated symphysis of the lower jaw.— 
Brandt.) 
RHYTINIDAE. (LXIII.) 
SUPER-FAMILY TRICHECOIDEA. 
LX. TRICHECHIDAE, 
Genus. 
Trichechus Linn=Manatus Storr=Oxystomus Fischer. 
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