﻿PTERYGOTUS BILOBUS. 



C3 



The following are the measurements of the body : 



Head, 1 inch 3 lines long, and 1 inch 7 lines broad. 



Segment 



3 / 



2 \ 



o 



o 



-a 



< 



Telson 



(8) 



2 



(9) 



3 



(10) 



3 , 



(11) 



2 



(12) 



2 



(13) 



n , 



(14) 



3 



(15) 



3 



(16) 



2 



(17) 





(18) 



3 , 



(19) 



7 , 



(20) 



11 



1 9 



2 inches broad. 

 2 



2 ,, 1 line broad. 



2 ,, 1 j, 



9 



1 inch 8 lines broad. 



1 >> 2 ,, 



1 ,, 1 line broad. 



1 1 lines 

 10 „ 

 9 „ 



In the example figured (PI. XI, fig. 1) the ectognaths or swimming-feet (ec), are both 

 in situ, and the impressions of their great serrated basal joints (s) can be seen through 

 the overhang head-shield. The thoracic plate, or operculum, with its pointed median 

 lobe, can likewise be seen impressed through the anterior thoracic somites. One of the 

 endognaths (<?) and one of the chelate antennas (a) are also seen detached from the head 

 and lying on the left side of the body. The large compound eyes (p, o) can also be seen 

 on the anterior border of the head-shield. 



The dilated form of this specimen appeared to me so very extreme as compared with 

 the great series of Pt. bilobus, var. a, which I have had the opportunity to examine, both 

 in the British Museum and the Museum of Practical Geology/ Jermyn Street, that I have 

 thought it at least entitled to a passing notice. 1 



Var. 3. — Pterygotus bilobus, var. y, perornatus. PI. XI, fig. 2, and Pis. XIII — XV. 



Himantopterus perornatus, Salter. Quart. Jonrn. Geol. Soc, 1856, vol. xii, p. 31, 



and p. 28, fig. 6. 

 Pterygotus perornatus, Salter. Mem. Geol. Surv., Mon. I, 1859, p. 45, pi. i, figs. 



13—15 ; pi. xv, fig. 2. 

 — — var. plicatissimus. lb., pi. i, fig. 16. 



In my report to the British Association for the Advancement of Science (read before 



1 In the 'Quarterly Journ. Geol. Soc.,' vol. xxiv, 1868, p. 295, I only spoke of three varieties of 

 bilobus, viz. var. inornatus, perornatus, and crassus ; it has since been found necessary to name a fourth 

 variety, h, acidens. (See also 'Report British Association,' Dundee, 1867, p. 44.) 



