84 CRUSTACEA—CIRRIPEDIA CHAP. 
Sub-Order 1. Pedunculata. 
In this division, sometimes combined with the Operculata 
as THORACICA, owing to the extremely reduced state of the 
abdomen, the body is borne on a distinct stalk, and the bivalve 
arrangement of the mantle is clearly retained. The mantle is 
protected externally by a number of calcareous plates, the 
arrangement of which is typical of the various genera. It 
appears that in the most primitive and geologically oldest 
Cirripedes, the probable ancestors of the Pedunculate and Oper- 
culate sub-orders, the arrangement of the plates was somewhat 
irregular, and they were far more numerous than in the modern 
forms, so that passing from these older types to modern times 
we witness a reduction in the number and a greater precision 
in the arrangement of the skeletal parts. 
One of the most ancient Cirripedes known is 7'wrrilepas, which 
‘occurs in the Silurian deposits of England, but it is also known 
from earlier deposits, while undoubted 
Cirripedes have been found in the Cam- 
brian of North America. The body of 
Turrilepas is enclosed in inbricating 
plates, as shown in Fig. 53, A. 
In Archaeolepas of the Upper Jurassic 
(Lithographic slates of Bavaria) the ar- 
rangement of scutes typical of the Lepa- 
A B didae is foreshadowed, but the whole 
Fie. 53. — A, Turrilepas : 5. ert, 
wrightianus (Silurian), x Of the peduncle is protected by rows 
1; B, Archaeolepas redten- of plates (Fig. 53, B), as in Turrilepas. 
bacheri (Jurassic), x 1. C, aa : : 
ane lee wedanae & The above-mentioned genera did not 
seutum; 7, tergum. (After survive into the Cretaceous period, their 
Zittel.) : 46 
places being taken by the genera Pollicipes 
and Scalpellum, which first appeared in the Silurian and persist to 
the present time, the older and more primitive Pollicipes being 
represented by about half a dozen living species, while the species 
of Scalpellum are exceedingly numerous. 
Fam. 1. Polyaspidae.—This family includes the three genera, 
Pollicipes, Scalpellum, and Lithotrya. 
Pollicipes is not only very ancient geologically (being found from 
the Ordovician upward), but it preserves the primitive character- 
