128 BRITISH FOSSIL CRUSTACEA. 



long tubercles borders the posterior margin, and the surface is covered with minute scale- 

 markings. The lateral portion is rounded and slightly expanded. 



The third segment is 2 inches wide and 10| broad. Three principal prominent 

 tubercles and several smaller ones mark the posterior margin, pointing backwards. 



The epimeral portion of all the thoracic segments is w T idely rounded, and a broad 

 margin of each segment overlaps the succeeding one. 



The fourth segment exposes 1 inch and 10 lines of its length. 



The tubercles upon this and the third segment are the most strongly marked of any. 

 It is 11 inches in breadth. The fifth segment (thoracic) is If inch in length, and 9 

 inches broad. The sixth segment is 19 lines in length and (about) 8 inches in breadth. 

 The seventh (or first abdominal) is 2 inches in length and from 5 to 6 inches in breadth. 

 (Here the specimen is fractured across, and the margins of this segment are wanting.) 

 Each of the abdominal segments has large epimeral pieces, which have been well preserved 

 upon the surface of the overlying slab. 



The eighth (or second abdominal) is 14 lines in length and 5 inches in breadth, 

 including the epimeral portion, which is clearly shown. The ninth (or third abdominal) 

 is 2^ inches long and 4| in breadth. The tenth (or fourth abdominal) is 2^- inches in 

 length and 4^ inches broad, including the epimeral portion. The eleventh (or fifth 

 abdominal) is 2f inches long and 3 inches 10 lines in breadth, including the epimeral 

 portion. The twelfth (or sixth abdominal) is 2£ inches in length and 3| inches 

 in width. Erom this segment two elongated epimeral pieces are developed, measuring 

 4^ inches in length by about 5 lines in width, and terminating in a broad rounded 

 point. 



The Telson (9J inches in length) is somewhat broad at the point of attachment, and 

 becomes slightly narrower in the first quarter, gradually widening to \\ inch. 



The central depression is f- inch in width. The termination is rounded, and the 

 border does not appear to have been ornamented. Of the appendages, there remains only 

 the joint of a swimming-limb attached to the left margin of the carapace, measuring 

 4 inches in length by 2 in breadth, and having a row of tubercles upon the centre. 



In a letter dated 9th February, 1865, Mr. Powrie wrote to me as follows : 



" Mr. Salter has expressed his conviction that Stj/lonurus Poioriei and S. Sections are 

 specifically the same — the larger one a full-grown male, and the smaller a young female, 

 the longer and narrower body, shorter tail, and epimeral appendages being all character- 

 istics of the male ; in other respects the resemblance is most marked." 



Were Ave to accept this opinion, that S. Powriei and S. Scotic/is are identical, then the 

 method of determining the sexes in the British and American species of Enrt/pterus, Ttery- 

 (jotns, and Slimonia, hitherto adopted, namely, by the two forms of thoracic plates, must be 

 abandoned. Moreover, if we are to be guided by more general characters than those of the 

 sexual plates, we must expect the antennae to be modified in the male, as in the recent 

 Limulns ; in which case the two or three forms of plates in Slimonia acuminata would 



