﻿64 BRITISH FOSSIL CRUSTACEA. 



Section C, at Dundee) in 1867, and printed in the volume for that year, p. 44, and also 

 in the 'Quarterly Journal of the Geological Society' for 1868, vol. xxiv, p. 294, I 

 pointed out the necessity of reconsidering the species of Pt. bilobus and Pt. perornatus. 



Previously to 1865 Pterygotus bilobus had been found by Mr. Slimon in a nearly 

 perfect state, being the first British species figured entire. 



It obtained its specific name from the bilobed character of the telson ; but at that 

 time no complete example of Pterygotus perornatus had been met with. 1 



Since then Pt. perornatus has also been obtained whole, and is likewise furnished with 

 a bilobed telson. 



Difference in size cannot be relied upon as a means of separating these two, nor can 

 the ornamentation be made use of as a specific distinction ; the appendages are alike in 

 both. 



The thoracic plate or operculum is valuable, no doubt, as a means of separating such 

 forms, but, unfortunately, it cannot often be seen. 



I have therefore concluded to retain the name bilobus as the specific designation for 

 all four forms described in this part, treating them as varieties only, until we can obtain 

 more positive evidence of their specific distinctness than we at present possess. 



That event arising, the specific name bilobus should be abolished, and the varietal 

 appellations retained as of specific value. I do not, however, apprehend that such a 

 necessity is likely to occur, the present tendency among naturalists being rather to 

 diminish than to increase the number of species. 



This is the largest form of Pterygotus found at Lesmahago, the almost entire 

 example — figured in PI. XIII, of one half of the natural size — being not less than 

 11^ to 12 inches in length and 3^ inches in greatest breadth. 



Pigs. 1 and 2 in PI. XV were parts of two individuals equally large, whilst the 

 detached head and swimming-feet figured of the natural size on PI. XIV would repre- 

 sent an animal 18 inches in length by 5 inches in breadth. If the detached chela (PI. 

 XI, fig. 3) belonged to this species, we have evidence of a form not less than 2 feet in 

 length and probably longer. 



Making a careful measurement of the body of Pt. perornatus figured in PI. XIII, and 

 aided by figs. 1 and 2 of PI. XV, we get the following proportions for an individual 

 about 12 inches in length: 



1 The head and first six thoracic segments of a specimen as large as that figured on our PI. XV are 

 drawn by Mr. Bone in pi. i of the ' Geological Survey Memoirs, Monograph I.' 



