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 P?. di/obus 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.’ 
Since then P¢. 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 dz/obus 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 2/odus 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 iieemehaea the almost entire 
example—figured in Pl. XIII, of one half of the natural size—being not less than 
11% to 12 inches in length and 34 inches in greatest breadth. 
Figs. 1 and 2 in Pl. XV were parts of two individuals equally large, whilst the 
detached head and swimming-feet figured of the natural size on Pl. 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 P¢. perornatus figured in Pl. XIII, and 
aided by figs. 1 and 2 of Pl. 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 Pl. XV are 
drawn by Mr. Bone in pl. i of the ‘ Geological Survey Memoirs, Monograph I.’ 
