A REVIEW OF THE MYSIDACEA 



23 



times as long as the antennular peduncle. Hansen's specimens were 

 "very mutilated and badly preserved," and this condition may account 

 tor the discrepancy. 



The four juvenile specimens in the collection show two interesting 

 i oints. Two of them lack the median rostral spine and the other two 

 nave it, yet they are all comparable in size. Hansen figures the rostral 

 plate of an immature specimen of P. glaber, 13 mm. in length, in which 

 the median rostral tooth is missing and suggests that this is a juvenile 

 character. One of the main characters upon which P. macrops was 

 founded by Colosi is the absence of the median rostral tooth. The 

 four known specimens of this species measure 9 to 13 mm. The inter- 

 pretation of this character is as yet a matter of doubt. It may be, as 

 Hansen suggests, a juvenile character; yet of the four young speci- 

 mens in my material, all smaller than any of the recorded specimens, 

 two possess the median tooth and two are without it. It may be a not 

 infrequent variation which persists throughout life, or it may repre- 

 sent, as Colosi appears to think, a definite specific character. 



The second interesting point shown by the young specimens is the 

 armature of the telson. They have only two or three small spines be- 

 tween the two large spines near the apex of the telson, whereas in fully 

 grown specimens of P. glaber there are six to seven such spines. I 

 have interpreted this difference as due to difference in size and age and 

 have supposed that the number of these spines increases with the 

 growth of the individual. If the known species of the genus are ar- 

 ranged according to size it will be seen that they differ in respect to 

 the number of small spines between the two large marginal spines at 

 the posterior end of the lateral margin in precisely the same way as if 

 they were stages in the growth of one species. P. sanzol differs from 

 the other species in the larger number of spines on the lateral margins 

 of the telson proximal to the large pair of terminal spines, eight as 

 against three or four in the other species. P. microps is separated on 

 the much smaller size of the eye. 



Table 3. — Species of Paralophogaster arranged to show ratio of small spines to 



size of species 



Species 



Number 

 of small 

 spines 



glaber 



intermedius 



sanzoi 



macrops 



microps 



atlanticus... 



6-7 

 5 

 3 

 4 

 4 

 3 



893476—51- 



