PTERYGOTUS BILOBUS. 63 
The following are the measurements of the body: 
Head, 1 inch 3 lines long, and 1 inch 7 lines broad. 
Segment (8) 2 3 9 :- 
” (9) 3 Ny 2 inches broad. 
c& By (10) 3 A 2 5 
g ell) .2 “ 2 ,, 1 line broad. 
é 3 2), 2 "3 a | nA 
Py, (13) 23 nf Dyan) a 
39 (14) 3 ks 1 inch 8 lines broad. 
: Baian(t5) 4:3 :, 1 Peapod hd 
E 2 CO ae 1 ,, 1 line broad. 
§ oe (lz) 22 ie Te ose aa ee 
= » (18) 3 ” aie 11 lines ,, 
» (19) 7 5 ainda" scene pe 
Telson (20) 11 ss gt .. 
In the example figured (Pl. XI, fig. 1) the ectognaths or swimming-feet (ec), are both 
im sitt, and the impressions of their great serrated basal joints (s) can be seen through 
the overlying head-shield. The thoracic plate, or operculum, with its pomted median 
lobe, can likewise be seen impressed through the anterior thoracic somites. One of the 
endognaths (¢) and one of the chelate antennz (a) are also seen detached from the head 
and lying on the left side of the body. The large compound eyes (0, 0) 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 P¢. di/obus, 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.! 
Var. 3.—PTERYGOTUS BILOBUS, var. y, perornatus. Pl. XI, fig. 2, and Pls. XIJI—XV. 
HIMANTOPTERUS PERORNATUS, Salter. Quart. Journ. Geol. Soc., 1856, vol. xii, p. 31, 
and p. 28, fig. 6. 
PTERYGOTUS PERORNATUS, Salter. Mem. Geol. Sury., Mon. I, 1859, p. 45, pl. i, figs. 
13—15 ; pl. xv, fig. 2. 
= _— var. plicatissimus. Ib., pl. 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, 5, acidens. (See also ‘Report British Association,’ Dundee, 1867, p. 44.) 
