NO. 2 MIDDLE CAMBMAN MEROSTOMATA 25 



extends backward from two-thirds to four-fifths of the distance 

 to the posterior margin; at about half its length a wing-like ex- 

 tension continues backward and slightly outward to a transverse 

 margin; on each side of the terminal segment (pi. 3, figs. 2-4), and 

 attached to its anterior side, there is a lateral swimmeret that on 

 the inside overlaps more or less the central terminal section and on 

 the outside margin expands so as to form, with the central terminal 

 section, a broad caudal fin suggestive of that occurring in the 

 Schizopoda and Decapoda ; it may be that there are more than one 

 of the lateral swimmerets on each side, but if so they are so pressed 

 in together as not to be distinguished. The anterior margin of 

 each segment extends under the segment in front of it from one- 

 fifth to one-third its length at the center, the underlap gradually nar- 

 rowing to where it passes from beneath the segment near its outer 

 termination ; the anterior segment passes beneath the cephalo-thorax 

 in the same manner. 



Ventral appendages: Cephalo-thorax. — Cephalo-thorax with five 

 pairs of movable appendages. The first or antennal pair are rather 

 stout at the base, tapering gradually until they become very slender 

 (pi. 2, fig. i) ; the joints vary in length, they are usually a little 

 wider than long for the first half of the length of the appendage, 

 gradually becoming proportionately longer toward the outer end. 

 One appendage shows over thirty joints beyond the edge of the 

 carapace, and another from its length must have many more ; each 

 joint has a short, fine spine or fringe of spines at the anterior margin 

 of the joint. The inner point of attachment of the first joint has 

 not been seen as it is covered by either the epistoma or cephalo- 

 thorax in all specimens. 



The second pair of appendages is formed of long, slender joints ; 

 so far as known they extend only a little distance beyond the margin 

 of the cephalo-thorax and terminate in a joint that has two or three 

 short spines projecting from the outer end of it. 



The simple form of the third pair of appendages is shown by 

 fig. I, pi. 5. They have a relatively large basal joint, the anterior 

 inner margin of which is provided with six or more sharp spines 

 that appear to have been used in connection with the basal joints 

 of the fourth and fifth pair of appendages as manducatory organs. 

 This appendage has broad, strong joints in small specimens (pi. 5, 

 fig. i) and in large specimens it is developed into a peculiarly con- 

 structed and complex chelate terminal section; this is formed of 

 twelve or more joints of a forward curving appendage to which 



