336 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



of the opposite side, and bearing on this margin a short, curved, styli- 

 forin organ. They are provided at the tip with stout pectinate setae, and 

 along the basal portion of the outer margin lies, on each side, the large 

 external lamella. The palpi of the rnaxillipeds are flattened and ciliated 

 along their inner margins, and the number of segments may be reduced 

 to three by the coalescence of the last two and of the preceding two. 

 The maxillae vary but little in the family ; the second or outer pair bear 

 as usual three delicate ciliated plates ; the first or inner pair are armed 

 with stouter setae and spines. The mandibles are robust, acutely toothed 

 at the apex, armed with a more or less powerful molar process, and 

 are destitute of palpi. 



The thoracic segments are distinct and subequal in length, but may 

 differ considerably in width, and are not united with the head nor 

 with the pleon. The legs, except in the genus CMridotea, are nearly 

 similar in form throughout, and, in the first three pairs at least, are 

 terminated by a prehensile or subprehensile hand, formed by the more 

 or less complete flexion of the dactylus upon the propodus. The first 

 pair of legs is usually shortest and has a triangular carpus. The an- 

 terior three pairs of legs are, in general, directed forward, and the 

 posterior four pairs are directed backward and are less perfectly, or not 

 at all, prehensile, a distinction that reaches its highest development in 

 CMridotea. The seventh pair of legs are absent in the young taken 

 from the incubatory pouch, and do not generally attain quite as large 

 size as the sixth pair. 



The pleon, seen from above, consists in great part, or entirely, of a 

 large, convex, usually pointed, scutiform piece, representing the con- 

 solidated terminal segments. As many as four of the anterior segments 

 may, however, be more or less completely separated by articulations or 

 indicated by lateral incisions or sutural lines. Underneath, the pleon is 

 provided with a structure peculiar to and characteristic of this family, 

 and the next, viz, a two-valved operculurn, formed by the specially modi- 

 fied uropods,* or appendages of the terminal segment, closing like a pair 

 of cupboard doors and protecting the delicate pleopods, which are lodged 

 in a vaulted chamber excavated in the under surface of the pleon. This 

 operculum consists, on each side, of an elongated basal plate, often strongly 

 vaulted, angulated externally near the base, where it is articulated with 

 the terminal segment of the pleou, and bearing at the tip one, or some- 

 times two, small lamellae. One of these lamellae usually disappears, but 

 two are present in Chiridotea, as also in the foreign genera Cleantis and 

 Chcetilia. When both are present the opercular plates differ only in 

 proportion from the ordinary form of uropods, consisting of a basal seg- 

 ment and two rarni. Within the cavity enclosed by the opercular plates 

 lie the usual five pairs of pleopods, each consisting of a basal segment 



* In the last edition of the Encyclopaedia Britannica (vol. vi, p. 641), these organs 

 are described as the " anterior " abdominal appendages. They are anterior only In 

 position, being in fact the appendages of the posterior segment. 



