464 PROCEEDINGS OF THE IS^ATIONAL MUSEUM vol. 88 



outward. These ribs are variously broken up into segments, which 

 are more or less characteristic for a given species. They may be a 

 series of long or short rods (fig. 26, c), segments simulating imbricate 

 plates (fig. 27, a), or an elongate segment followed by a series of 

 overlapping plates (fig. 22, h). The number of plates in the ribs 

 varies only slightly for a given species. They will be used for identi- 

 fication purposes. 



The next pair of appendages constitutes the second maxillae. They 

 have a triangular plate on the basal segment, which is divided pos- 

 teriorly into three teeth or lobes (fig. 26, a). The shape of these is 

 more or less constant for the species. On each side of the midline 

 of the body at the base of this appendage is a pair of spines, and 

 posterior to it, just ahead of the first thoracic appendage, is a second 

 pair. These two pairs of postmaxillary spines will be used along 

 with the teeth of the basal plate to aid in identification. These ap- 

 pendages are considered maxillae because they occur ahead of the 

 tranverse groove separating the head and thorax and have an excretory 

 gland, the so-called maxillary gland at their base, and because no 

 maxillae are found in the buccal cavity as sometimes stated. 



The four pairs of thoracic appendages are made up of a protopod, 

 composed of the precoxa, coxa, and basis, and of two rami, the exopod 

 and endopod. The precoxa is very small in most species, but on a 

 few with long appendages it becomes sizable. The two anterior 

 pairs may have a flagellum extending mesially from its point of 

 origin (fig. 38, d) on the dorsal side at the base of the exopod. 



On the female the thoracic appendages may be modified by having 

 a slight chitinous ridge armed with setae on the posterior ventral 

 surface of the coxa. Almost every species has a boot-shaped or 

 rounded natatory lobe extending from the posterior of the coxa of 

 the last appendage. On the male it may or may not be the same size 

 and shape as that of the female of the same species. 



On the male the second, third, and fourth appendages are modified 

 for copulation. These specific modifications have hitherto been used 

 as practically the only positive means of identification. On the 

 ventral posterior axis of the second appendage is usually found a 

 bilobed prominence extending the full length of the coxa. It may 

 be variously modified or be entirely lacking. This structure is usually 

 covered with chitinous spines, which are very often truncate. 



Typically the coxa or basis of the third appendage is modified on 

 posterior surface by the formation of the socket (fig. 32, c), the so- 

 called "semen capsule" or "semen pocket" of many authors. This is 

 formed by a fold of tissue from the posterior or ventral side of the 

 appendage turned toward the dorsum to produce a pocket. From 

 the ventral side it appears as a rounded or oblong lobe projecting 

 backward and overlaps the "peg" on the next appendage. From 



