334 The American Naturalist. [April, 



discovory in Steramatoiulus. In that form, however, the multiplicity 

 of peculiar characters weakens the application of homologies to the 

 other orders, unless these can be based on structural facts. It is thus a 

 matter of interest that the existence of pleurae in another Diplopod 

 order can be affirmed. 



Some weeks since in exu >pir<>holid;c I noticed 



what seemed to be traces of pleural sutures. On mentioning this fact 

 to Mr. F. C. Straub who was studying with me, he called my attention 

 to a specimen of Spirobolus marginatus Say which proved to be very 

 remarkable. Possibly it was collected just after moulting, before the 

 sclerites had become coalesced, or it may have been merely an individ- 

 ual anomaly. At any rate, it had on each side an obliquely longitudi- 

 nal white line across each segment above the pedigerous lamina, 

 indicating a pleural element about as broad as the lamina. That this 

 is the pleural suture seems very probable, on theoretical grounds and 

 more so that on the surface a special strife followed the line of white. 



What is more remarkable, this line was met above by two others 

 which were transverse, dividing the segment into three subequal part*. 

 These two lines extended completely over the animal, the space between 

 them being somewhat greater above. There is also a median longitu- 

 dinal suture, and a lateral just below the pore, thus dividing the dorsal 

 portion of the ring into twelve subequal parts. The posterior of the 

 transverse sutures follows the depression found in the segments of Spi- 

 roboli and usually called " the suture" in descriptions. The anterior 

 line and the median line are indicated by minute differences in the 

 sculpture, which would not have been noticed had not the white line 

 drawn attention to them. It should be added that the lighter color 

 was not due to anything inside or outside the segmental wall, but was 

 in the wall itself and clearly indicated some structural difference. The 

 phenomenon was exhibited by the anterior and middle segments of the 

 body, becoming indistinct caudad. In all cases the pattern was the 

 same; the whole series of lines could be made out on many segments, 

 and there were no other similar lines or discolorations. The lines were 

 not straight if examined uiidm- a microscope, even the median sli.iwing 

 minute irregularities. Median sutures are known in four or five of the 

 Diplopod orders and hence may reasonably be expected in all. 



Had only the median line been marked as related, there would have 

 been no hesitation in supposing that a median suture yjas indicated. 

 Theoretical considerations only stand in the way of the reasonable 

 presumption that the other exactly limilar linei indicate sutures. If 

 such an interpretation is allowed we are brought to the position that 



