24 
Genus — PENTAMERUS, Sowerhy. 
1. Pentamerus Knightii, Sowerhy. 
Tentamerus Kniglitii, Sowerby, 1813, Min. Concb., I, p. 73, p]. 28, fig. 1. 
,, Aylesfordii, Idem, id., Ibid., I, p. 75, pi. 28, fig. 2 and pi. 29, fig. 1-3. 
,y Kniglitii, Davidson, 1871, Mon. Brit. Sil. Bract., p. 142, pi. 16, fig. 1-3, 
pi. 17, fig. 1-10, et pi. 19, fig. 3. 
The form and the characters of this species are so well known, and it 
has been so well described and so copiously illustrated by my learned friend, 
Mr. Davidson, that it appears to me to be needless to enter into details about it. 
I will confine myself to saying that several varieties of it have been 
collected by Mr. Clarke. One of these resembles that figured by Davidson in 
his PL xvii, Eig. 7 ; another has considerable analogy to his Eigure 4, and a 
third with Eigure 5 of the same plate. Part of a large individual showing the 
internal septa is so like the specimen shown in PI. xvi, Eig. 5, that one would 
take a part of this figure to be a drawing of this fragment. Consequently, 
there can be no doubt about the determination of this species. 
Horizon and Localities. — This species is not uncommon in the Upper 
Silurian of England, Scotland, Ireland, Bohemia, and Russia. Many specimens 
have been collected in Australia, at Calalamine, in a greyish limestone, and in 
a white limestone from the Canobolas. 
2. Pentamerus oblongus, J. Sowerhy. 
L entamerUS laevis, J. Sowerby, 1813, Min. Concb., I, p. 76, pi. 28, fig. 3. 
,, ohlongus, J. de C. Sowerby, 1839, in Murcbison’s Silurian System, pi. 19, 
fig. 16. 
„ „ Salter, 1859, in Murcbison’s Siluria, p. 544, pi. 8, fig. 1-4. 
„ „ T. Davidson, 1871, Mon. Brit. Sil. Bracb., p. 151, pi. 18, fig. 1-12, et 
pi. 19, fig. 1, 2. 
This s 2 oecies, generally oval in form and with quite smooth surface when 
young, is very variable when adult. The ventral valve is divided in two by a 
median septum, which occupies two-thirds of the length of the valve, and 
which serves as a suj)port for the dental plates. The dorsal valve also has two 
' [Vide Etheridge, R., Junr. — T he Pentameridse of New South Wales, Records Geol. Survey of N. S. 
Wales, 1892, iii, pp. 49-60, pis. 10, 11.— W.S.D.] 
