1878.] THE ANATOMY OF PLOTUS LEVAILLANTI. 681 



corresponding to the surface between the limbs of the U, apparently 

 not glandular in nature. All these structures are covered with the 

 tough gastric epithelium, which ceases just above them. 



The first true gastric cavity is larger than the second, though not 

 much so. In the second the peculiar hairy covering of its pyloric 

 portion is largely developed, and in a different manner from what it 

 is in P. anhinga, where, as f have shown in my paper on the anatomv 

 of that bird, it forms a kind of sieve to prevent large solid particles 

 from entering the duodenum. In P. levaillanti a more elaborate 

 arrangement obtains ; the hairy epithelium surrounding the pyloric 

 orifice, near the lower margin of the gastric surface of which it is 

 developed, is produced into a considerable conical hair-covered pro- 

 cess, projecting into the second stomach, and evidently acting as a 

 valve to close the pylorus when necessary. In general appearance 

 it much resembles the operculum of the Cheilostomatous Polyzoa, 

 and is very striking at first sight, the hirsute conical plug when re- 

 tracted, fitting exactly into the equally hirsute conical pyloric end of 

 the second stomach-cavity. All the rest of the second stomach is 

 lined with a non-hirsute epithelium, which ceases abruptly where 

 it meets the hairy surface. I can find no trace of this operculum in 

 Plotus anhinga, upon re-examination. 



The small intestine measured two feet, and the large three inches ; 

 but they may have been contracted by the inflammation of their 

 surfaces. Two minute caeca were clearly seen, one a little larger 

 than the other. In P. anhinga there is no indication of a second 

 caecum. 



As in P. anhinga, P. levaillainti possesses but one carotid artery, 

 the left. In their myology the two species agree in every respect, 

 as far as I can see. In P. levaillanti the ambiens is large, groov- 

 ing the patella, the femoro-caudal is present without an accessorius, 

 as is the semitendinosus. There is a slip from the biceps of the 

 arm, which traverses the patagium ; and the temporal muscles run 

 back beyond the skull, being separated by a median fibrous raphe, 

 which is not ossified into a separate bony style. The great pectoral 

 muscle is formed of two layers. 



Donitz's bridge is ossified, as in the specimen described by the 

 author after whom it is named : it is developed on the ninth, and 

 not on the eighth cervical vertebra, as I predicted would be the case. 



The lower larynx is indistinguishable from that of P. anhinga. 



It is interesting to notice that the Manatee and Dugong have 

 special gastric gland-structures, the method of arrangement of which 

 differs in exactly the same way as does that of the two species of 

 Plotus under consideration, the peculiar flat gland-area found in 

 Halicore and Plotus levaillanti being converted into a glandular 

 cavity in Manatus and Plotus anhinga. 



Proc. Zool. Soc— 1878, No. XLIV. 44 



