32 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol. 60 



ment. Spermatophore receptacle large and near the posterior end 

 of the genital segment. 



The distinguishing character of the genus is a closed vascular sys- 

 tem made up of two ventral longitudinal trunks running under and 

 close to the intestine, one on either side, and a single dorsal trunk 

 over the intestine and between the paired sex organs. From these 

 central trunks branches lead to the A^arious appendages and there 

 is also a network of capillaries over the dorsal surface and in the 

 laminate swimming legs. No part of the system has any connection 

 with the lumen of the body cavity. The trunks and capillaries are 

 filled with a yellowish-red liquid, which streams back and forth 

 under the influence of the peristaltic movements of the digestive 

 canal, and aided by the contraction of the muscles of the various ap- 

 pendages and body regions. This liquid contains neither blood cor- 

 puscles nor any other definite constituents, and hence can not be 

 called true blood, but it probably serves as an oxygen carrier between 

 the body regions and into the appendages. 



Type of the genus. — Lemathropus musca Blainville, 1822, mono- 

 typic. 



KEY TO THH SPECIES. 



(The number preceding the species name represents the total length of that sex and 



species.) 



1. A ciorsal plate covering the free thorax and genital segment, but leaving the 

 abdomen visible in dorsal view 12 



1. A dorsal plate covering the entire body, leaving nothing visible except the 

 rami of the third and fourth legs 2 



1. No dorsal plate, the free thorax, the genital segment and the abdomen 



entirely visible in dorsal view 30 



2. Third legs folded in the usual manner and projecting at right angles or 



obliquely to the ventral surface 3 



2. Third legs flattened into broad laminae parallel with the ventral surface 

 and covering nearly the whole of it 24 



2. Third legs narrow laminae, uniramose or divided and lying flat on the ven- 



tral surface, but covering only a little of it 27 



3. Dorsal plate all one piece, with no transverse groove or marginal sinuses i 



3. Dorsal plate divided into an anterior and posterior portion by a transverse 



groove or by marginal sinuses 8 



4. Males, dorsal plate no wider than the cephalothorax ; third and fourth legs 



projecting well beyond its margin 5 



4. Females, dorsal plate but little wider than the cephalothorax, and about the 

 same diameter throughout 6 



4. Females, dorsal plate widened posteriorly to twice the diameter of the 



cephalothorax 7 



5. Cephalothorax longer than dorsal plate ; latter obovate, much narrowed 



posteriorly 1 mm., male, larvatus Heller, 1S65 



5. Dorsal plate much longer than cephalothorax, not narrowed, but squarely 



truncated posteriorly 1.50 mm., male, laiiventris Heller, 1865 



5. Cephalothorax and dorsal plate about the same length, the latter broadly 



rounded posteriorly 1.50 mm., male, Jtolmbergii Nordinann, 1864 



