4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



The peculiar location of the genital pore in the jMystacocarida 

 represents an unusual condition. In the great majority of Crustacea 

 the genital pore is at or near the posterior end of the thorax. Indeed, 

 according to the Caiman scheme, the thorax is defined as that region 

 behind the head and in front of, and including, the segment which 

 bears the male genital pore ; also, with the exception of the hermaph- 

 roditic Cirripedia, the female genital pore is on the last or third-last 

 thoracic segment. Such a definition is arbitrary, to be sure, but it 

 is in striking contrast to the situation in the Mystacocarida. 



In spite of the numerous significant morphological differences be- 

 tween the Copepoda and the Mystacocarida, the similar segmentation, 

 the general body organization, and the number and arrangement of 

 the appendages constitute convincing evidence that these two ordei'S 

 are very closely related. Certainly it does not seem possible to demon- 

 strate important phylogenetic affinities between the Mystacocarida and 

 any other order. 



Undoubtedly, the Mystacocarida are the most primitive living 

 Crustacea which have thus far been discovered. In addition to the 

 simple body organization, this is clearly indicated by the persistence 

 of larval characters in the head, including the unspecialized primitive 

 morphology of the large mouthparts, the labrum, and the rostral 

 shield. The lamellar structure of the abdominal appendages, on the 

 other hand, is obviously a reduced condition. 



DEROCHEILOCARIDAE, new family 



Characters as of the type genus, Derocheilocaris 



DEROCHEILOCARIS, new genus 



Body elongate and cylindrical; all segments clearly marked off 

 from each other; thorax and abdomen about the same diameter 

 throughout; head and postcephalosome distinctly narrower than rest 

 of body. Head covered with a cephalic shield which has a large 

 rostral plate bearing a prominent, notched rostrum. Anal segment 

 with a large papilla dorsal to the anus. A pair of lateral, dorsoventral 

 troughs with heavily chitinized edges on the posterior part of the 

 cephalic shield and on all other segments except the last. 



First antennae 8-segmented, the terminal segment longest. Second 

 antennae tactile and natatory; both rami elongate. Both rami of 

 mandibles elongate, but endopod distinctly shorter than exopod ; 

 gnathobase on basipod. Instead of projecting ventrally, the second 

 antennae and mandibles are flexed so that they extend dorsally and 

 laterally away from the median plane of the body. Labrum very 



