256 ELYSIID.E. 



head distinct, obtuse, sliglitly emarginate ; anterior angles of foot widely pro- 

 duced, triangular. 



Artceon , Agassiz, Proc. Bost. Soc. Nat. Hist. iii. 191 (1850). 



Actccon chloroticus, Agassiz, in iMSS. 



Animal emerald green, finely dotted with opaque white inter- 

 spersed with red species. Body slender, tapering backwards, with 

 very broad lateral expansions or wings, which, when folded as they 

 are when the animal is crawling, overlap each other on the back in 

 a roof-like manner, and the whole animal has then a lance-shaped 

 form generally acutely pointed behind, but in some attitudes obtuse ; 

 when expanded, they have a broad ovate form, like a leaf with the 

 border more or less undulating, and this resemblance is further car- 

 ried out by the vein-like folds or canals which ramify on its surface 

 from the heart which forms a globular or bulbous eminence in front ; 

 the expansion begins at the anterior part of this bulb. In front of 

 this is a well-marked neck and head, on which latter are two deli- 

 cately lanceolate tentacles, which are furrowed or folded beneath. 

 The eyes are placed a little behind the tentacles. The head is ob- 

 tuse and slightly emarginate. The organs of generation are just 

 behind the right tentacle, and the male organ is very often pro- 

 truded, of about the same form and nearly as large as the tentacle. 

 The anterior angles of the foot are widely produced, of a recurved 

 triangular form, as if another pair of tentacles. Length, about one 

 inch, sometimes an inch and a half ; breadth, when folded, about 

 one fifth the length, and height equal to breadth, when fully ex- 

 panded, equal to three fourths tlie length. 



Found in great numbers in brackish water, on the Cambridge 

 marshes, in the spring of 1848 (^Ag'ctssiz'). 



Genus PLACOBRANCHUS, Van Hasselt. 1824. 



Body with two large, semi-circular, membranous expansions, 

 crossing on the back and forming a canal open at both ends. Ten- 

 tacles club-shaped, lobed at the end. 



Placobranclius catulus. 



Plate XVII. Pigs, 249, 250. 



Animal sea-green Avith Avhitish spots ; body ovate-lanceolate ; lateral expansions 

 two thirds its length, not meeting when reflected over the back ; head large, 

 rounded, globose; tentacles short, blunt, broad; foot wide as body, square in 

 front, pointed behind. 



