192 Dr. M. I. Newbigin on 



distance between the annuli, the specimens did not display 

 the constancy indicated by St. Joseph's descriptions of Dinard 

 specimens. Not only was there variation in the different 

 bristles of the same worm, but the same bristle often gave 

 varying figures for its different annuli. The tollowing are 

 some typical measurements: — 0'02l millim., 0"034 millim., 

 0-036 millim., 0-0;39 millim., 0-057 millim. The dorsal 

 bristles usually numbered 5-6 instead of 4—5, and not infre- 

 quently in addition there were one or two small bristles 

 embedded in the substance of the papilla and not projecting 

 from its surface. There was a wide range of variation in 

 regard to the distance of the annuli apart (see PI. IV. Hg. 5). 

 The total length of the bristles varied slightly in the different 

 specimens; the longest was usually 1*6 millim. to 1*2 millim. 

 in length ; this agrees very closely with St. Joseph's descrip- 

 tion for the Dinard specimens. In regard to the ventral 

 hooks and their associated simple bristles, the specimens 

 agreed closely with St. Joseph's description, but the minute 

 characters of the hooks did not entirely agree with his account 

 (see fig. 6). 



In general the Plymouth specimens, though agreeing very 

 closely with St. Joseph's Dinard specimens, showed variation 

 in three points : — (1) the number of cephalic bristles, (2) the 

 number of notopodial bristles in the other regions of the 

 body, (3) the minute structure of the bristles. All three are 

 characters which, according to St. Joseph, enter into the defini- 

 tion of the species. 



The Millport specimens of Siphonostoma showed a great 

 variation in size, some reaching a length of 70 millim., with a 

 breadth of 7 millim., and others being only 25 millim., witli a 

 breadth of 3 millim. All were taken in the same locality 

 (neighbourhood of Tan Buoy) , and all from the interior of 

 Lima nests. As large and small specimens occur together in 

 the same nest, it seems legitimate to conclude that the latter 

 are merely young stages of the former. 



The large specimens — those of 60-70 millim. in length — 

 are readily distinguished from the Plymouth species. As 

 obvious distinguishing features we have not only the length 

 of the body and the breadth of the anterior region, but the 

 more distinct tapering of the posterior region and the large 

 size of the notopodial bristles, which project in a very con- 

 spicuous way. Specimens of 25 millim. in length, on the 

 otiier hand, show a much greater resemblance to the Plymouth 

 form. We shall consider separately the characters of the large 

 and small specimens, but it should be noted that specimens 



