232 
S1LUKIA. 
[Chap. X. 
to have formed the chief diet of numerous Encrinites of the period, both in 
England and America (see p. 223). Natica ? parva, PI. XXV. f. 1, and a few 
other shells, complete the list of published species; but many others, I am 
assured by Mr. Salter, remain to be described. 
One or two Pteropods only have yet been detected ; nor do they seem to be so 
plentiful as in the lower rocks. Theca Forbesi of Sharpe, a species very like 
that figured in Foss. 41. f. 1. p. 199, and Th. anceps, Salter, are Wenlock Shale 
species, and the former is also common in the Upper Wenlock rock. Besides 
these, Conularia seems to be the only British Upper Silurian Pteropod, unless 
Ecculiomphalus lsevis, Sow., PI. XXV. f. 9, may be considered a member of 
this group. The beautiful and variable Conularia Sowerbyi, Defr. (C. quadri- 
sulcata, var., Sil. Syst), PI. XXV. f. 10, is often found in Wenlock Limestone, 
and occurs with a rarer species, 0. subtilis, Salter, in the Ludlow rocks of West- 
moreland, 
Bellerophons are frequent. Bellerophon dilatatus, Sow., PI. XXV. f. 5, 6, is 
one of the largest Nucleobranchs in the Ludlow and Wenlock rocks. The broad, 
expanded mouth is often three inches wide, and is sometimes furnished with 
radiating ribs (see fig. 5),— at other times smooth, Foss. 41. f. 8, p. 199. B. Wen- 
loekensis, Sow., PI. XXV. f. 7, is very characteristic of the strata implied in its 
name. B. expansus, Sow., f. 8, is equally so of the Upper Ludlow rock. The 
latter and B. Murchisoni?, d'Orb., PI. XXXIV. f. 19, B. carinatus, Sow., f. 8, 
and B. trilobatus, Sow., f. 9, generally of small size, are most abundant every- 
where in the Upper Ludlow rock. 
Of the Cephalopods which chiefly typify the Upper Silurian strata, it is un- 
necessary to repeat what has been said in describing the Wenlock and Lud- 
low formations ; the species which most abound have already been enumerated 
(see pp. 114, 127, &c). It may, however, be noted that some of these forms 
are specially characteristic of particular strata. Thus the thin-shelled species 
of Orthoceras — 0. subundulatum, Portl., and 0. primsevum, Forbes, here figured, 
are the most frequent shells of Wenlock Shale; 0. annulatum, PI. XXVI. f. 1, 
and its variety, fimbriatum, f. 2, is a well-known Wenlock Limestone species; 
Fossils (62). Uppjer Silurian Cephalopoda. 
J 
1. Orthoceras filosum, Sow. 2. O. Ludense, Sow. These species are figured from 
specimens two feet in length. 3. O. subundulatum, Portlock (Creseis Sedgwicki, Forbes). 
4. O. primsevum, Forbes. The two latter are usually 8 or 9 inches long. 
O. filosum, PI. XXVII. f. 1 (Foss. 62. f. 1), O. ? perelegans, PL XXIX. f. 5, 6, and 
O. dimidiatum, PL XXVIII. f. 5, are common Lower Ludlow fossils ; O. Mock- 
treense, PI. XXIX. f. 2, is a Middle Ludlow or Aymestry species ; while 0. Lu- 
dense, Foss. 62. f. 2, and PL XXVIII. f. 1, O. angulatum, f. 4, 0. ibex, PL XXIX. 
f. 3, and especially O. bullatum, f. 1, are frequent Upper Ludlow forms. 
Actinocerasnummularium,Pl. XXVI. f. 5, Orthoceras excentricum, PL XXVII. 
