12 C. Ü. Whitman 



Total uumberofectodermalcells 27. 



Lengtli of the long- est measured, 4mm; mostofthe 

 1 arger specimen 8 measure 2 — 3mm. 



Van Beneden has given no figures of this species, but has set up 

 the following as differential characters. 



1. The calotte is much more oblique than in D. ti/pus, and its 

 obliquity is attributed wholly to the ventral cells being smaller than the 

 dorsal. 



2. The propolars are conspicuously larger than the metapolars, 

 and in this respect the species is said to differ from all others. 



In the earlier cases examined , I found the propolars not larger, 

 but, in many cases at least, smaller than the metapolars (figs. 30 and 33), 

 and was naturally surprised to find my observations thus clashing with 

 those of Van Beneden. I was equally unfortunate in not being able to 

 confirm his Statements in regard to the obliquity of the head. Later 

 (Apr. 15) I obtained some material which served to explain , in a meas- 

 ure , these discrepancies. The calottes of Ihree of the larger individuals 

 are outlined in figures 31 . 32, and 34. Fig. 34 agrees with statement 

 No. 2, but shows little or no obliquity. In figs. 31 and 32 the obliquity 

 is very well niarked, but not so extreme as some cases noted in D. typus. 

 The propolars [avp and adp, • seen in optical section (fig. 32) , appear 

 larger than the metapolars ; but as the latter are considerably longer 

 than the former , the actual difference in volume is certainly not great. 

 A general correspondence between the degree of obliquity and the size 

 of the individuals was observable ; but not infrequently individuals of 

 approximately the same length were to be seen lying side by side, some 

 showing no obliquity, others exhibiting it in diflferent degrees. Had my 

 study of this species not extended beyond the examples obtained on this 

 occasion, I should have been able to say that the majority, both among 

 the larger and the smaller , show more or less obliquity ; and that in 

 most cases the propolars are nearly equal to the metapolars , in a few 

 even larger. But in my efforts to find specific distinctions that could be 

 Said to be decisive in all cases , I have been apprised , over and over 

 again, of the fact, that the obliquity of the calotte a'nd the 

 inequality of its cells are extremely variable, andthat 

 the ränge of variability can seldoni or never be ascertained 

 from examples t a k e n f r o m a s i n g 1 e Cephalo pod. This fact 

 makes the task of describing very laborious, since the characters found 

 to hold good in a single case must be revised and supplemented several 

 times before they can be safely regarded as general rather than exceptional. 



