﻿130 BRITISH FOSSIL CRUSTACEA. 



preserved, yet the specimens are so compressed as to give scarcely any idea of their original 

 contour, except by comparison with those from the Old Red Sandstone of Forfarshire and 

 elsewhere. 



The Byes are situated upon the surface of the carapace, somewhat near the anterior 

 angles. They were reniform, and raised upon round prominent bosses, but these are 

 now squeezed flat to the surface of the head. 



The Carapace is quadrate, with the anterior angles rounded ; the sides present a slightly 

 waving outline, contracting towards the posterior angles. The margin is double, having 

 an inner ridge, which circumvents the sides and front and terminates in a rounded 

 elevation at the posterior angles. An inner border-line also passes up each side and around 

 the front of each eye, reminding us of the cheek-sutures in the Trilobites. 



No sculpturing is noticeable upon the surface of the carapace, but (as is the case with 

 all the Logan Water specimens) impressions of portions of the organs are seen, the 

 position of which is beneath the surface of the head. 



The basal joints of several of the palpi are visible; and, underlying the 1st and 

 2nd thoracic segments, may be discerned the median appendage of the thoracic plate. 



The Body is extremely slender, as compared with Pteryyotus, and the segments in this 

 species (as in St. Powriei) had very small epimeral portions. The 11th and 12th segments, 

 which are not preserved, were doubtless still narrower ; and the telson was probably a long 

 styliform appendage, as in St. Powriei. 



Dimensions of Swimminy-foot .- — Basal joint, 6 lines in length and 5 broad; 2nd 

 joint, 2 lines long, 4 broad ; 3rd, 1 inch long, 2 lines broad; 4th, 10 lines long, 2 lines 

 broad ; 5th and 6th, each 8 lines long and 2 lines broad ; 7th, 6 lines long, 1 line broad ; 

 8th, 5 lines long, 1 line wide, terminating in a fine point. 



Carapace .- — Greatest anterior breadth, 1 inch 3 lines ; greatest length, 1 inch 

 2 lines ; width between the eyes, 5 lines; breadth of inner raised margin, 1 line. 



Thoracic seyments : — 1st segment, 2 lines long and 1 inch and 1 line wide ; 2nd and 

 3rd, 3 lines long and 1 inch wide ; 4th, 4 lines long and 1 inch 1 line wide ; 5th, 3 lines 

 long by 1 inch wide ; 6th, 2 lines long and 11 lines wide. The posterior margin of each 

 segment is ornamented by a row of minute spines along the border. 



Abdominal seyments: — 7th segment, 2 lines long by 10 lines wide; 8th, 3 lines long- 

 by 9 lines wide ; 9th, 3 lines long by 8 lines wide ; 10th, 3 lines long ; — here the specimen 

 is broken, and the 11th and 12th segments and telson are wanting. 



The form of the carapace and the position of the eyes are two very well-marked features 

 in this genus ; these, and the two pairs of long slender oar-like feet, sufficiently separate 

 them from the rest of the family, but even the long tail-spine is peculiar. 



From the extreme rarity of its occurrence in a formation where other genera are so 

 numerously represented, I am strongly inclined to believe this form to have been a larval 

 condition probably of some other genus of the same group. 



I have named it after my friend Sir William Logan, F.R.S., the late Director of the 



