williams.] THE CHARAOTEEISTIO SPECIES. 85 
which closely resemble each other in other respects, gives reason for 
supposing that the British form is at least represented by these Gaspe 
and Maine species. 
In Hall's description of the Arisaig specimens of Chonetes nova- 
scotica it is stated: ''A stronger and more elevated stria often marks 
the median line from beak to base of the ventral valve." 1 This feature 
is seen on several of the specimens of the Chapman sandstone, and, 
curiously, it is not always the central plication. In one case several 
plications, a little out of the center, are thus enlarged. Such a trick 
of variation would seem distinctly to indicate close phylogenetic 
relationship. 
In the Oriskany of Albany and Schoharie counties, in New York, a 
form (O. complanatus) quite as large as Billings's C. canadensis is 
described by Hall. The larger forms are all from the Gaspe, Maine, 
and Oriskany faunas at the summit of the Silurian. Zeptwna lata, 
which is figured as larger than the ordinary types of Chonetes, is said 
to be "one of the most characteristic shells of the Upper Ludlow." 2 
The bellerophons furnish a second set of diagnostic species. In the 
original list of the Tilestone fauna four species of Bellerophon are 
named, viz, Bellerophon carinatus, B. striatus, B. trilobatus, B. glo- 
batus. 
Bellerophon trilobatus is said to be a characteristic form of the Tile- 
stone. 3 This species is represented in both the Chapman sandstone and 
formation D of Arisaig. In Siluria it is stated that " B. expansus, 
B. Murchisonce, B. ca/rinatus, and B. trilobatus, generally of small 
size, are most abundant everywhere in the upper beds of the Ludlow 
rock" (p. 231). Belleropho?i carinatus and B. triloba his are both rep- 
resented in the Chapman; and Honeyman reports all four species from 
the Upper Arisaig, Zone D. 4 
A third diagnostic form is called u Agnostics tuberculatus {Battus 
tuberculatus Kloden)" by Sowerby in 1839. 5 This is Beyrichia tuber- 
culata Kloden. Several specimens of this species, or a very closely 
related form, are recognized in the Chapman fauna. Hall described, 
under the name Beyrichia pustulosa, a species from Zone D of the 
Arisaig, of which he says: "This species resembles very nearly the 
B. tuberculata of Kloden, as described and figured by Mr. T. Rupert 
Jones." Of this form Murchison writes, in Siluria, as follows: "Of 
the latter genus the Upper Silurian species is B. tuberculata. It 
is very abundant from the Wenlock shale to the highest Ludlow 
1 Descriptions of new species of fossils from the Silurian rocks of .Nova Scotia, by James Hall: < !ana- 
dian Nat. and Geol., Vol. V, p. 145. 
2 Silurian System, p. 603. 
3 Op. cit., p. 141. 
4 Quart. Jour. Geol. Soc, Vol. XX, p. 343. 
5 Silurian System, p. 604, PI. Ill, fig. 17. 
6 See Jones's identification of this species in the Arisaig rocks, in Quart. Jour. Geol. Soc, Vol. XXVI, 
p. 492. 
