544 TRANS. ST. LOUIS ACAD. SCIENCE. 



Fig. 4. ^ 



Transverse sections of ambulacral fields, to show abnormal developments of the 

 hydrospiric sacs: A. oi Pentrtmilres pvroformis ; B, P. forcalis, C, P.conoidens — 

 about 20 times magnified and drawn with the aid of the camera lucida. a, hydrospiric 

 sac; b, calcareous part of ambulacral field, i.e. lancet and poral pieces. 



Now, if we consider the second point, i.e. the external open- 

 ings, the so-called spiracles, we will find that Mr. Carpenter is 

 very inconsequent in his argument ; for he says, " mere differ- 

 ences in the relative sizes of the calyx plates are of very little sys- 

 tematic value," etc. But, it seems, Mr. Carpenter forgets that 

 the differences in the external openings are caused by the very 

 differences in the relative sizes of the deltoid and lancet pieces ; 

 or does he mean to intimate that these pieces do not belong to the 

 calyx plates ? 



Still, if we suppose such a classification, according to the dif- 

 ferences in the spiracle openings, was desirable and necessary, 

 we could only (according to their external aspect and arrange- 

 ment) divide them into three divisions, viz. : The first division 

 would comprise all those species in which the horizontal portion 

 of the deltoid piece is very narrow, the sinus to both sides in the 

 deltoid and lancet pieces comparatively large, and so surrounded 

 by the zigzag plicated integument that two of the so-formed open- 

 ings appear externally only as one (see Fig. 5, a). This division 

 would embrace such species as 



Pentremttes florealis Say, 



" sulcatus Roemer, 



" fyrifonnis Saj, 



" Wortheni Hall, 



" Rein-vardtii Troost, 



and others. 



The second division would comprise all those species in which 

 the deltoid pieces are very broad, the lancet pieces very narrow* 

 and the sinus for the formation of the spiracle openings in both 

 deltoid and lancet pieces very little ; the zigzag plicated integu- 

 ment corresponding to the narrow ambulacral field is not wide 



