143 
delations and Differences. — Before having an opportunity of studying 
it in nature I had entertained many doubts as to the specific value of this 
Droductus. Now I entirely agree with Meek and Worthen, who consider 
it a perfectly distinct species. P. giganteiis and P. semireticulatus, Martin, 
most nearly approach this species, and it occupies, so to speak, an intermediate 
place between them. It resembles the first by its general form, by its great 
size, and by the row of spines with which the hinge line is covered, hut differs 
from it by the thinness of its test, by the absence of strong muscular 
impressions, and above all by the absence of the two depressions whieh give 
rise to the two large protuberances that characterise the internal moulds of 
full grown specimens of P. gigantem ; again, its ears are never turned round 
like a horn, and its dorsal valve, far from following the contour and curve of 
the ventral valve, as in P. gigantem, deviates from it and is level for the 
greater part of its length, thus leaving a greater space for the soft parts of 
the animal. It cannot be confounded with P. semireticulatus, of which I 
formerly considered it a variety, because its ventral valve is never geniculated 
whatever its size and age ; while the visceral part is never completely 
separated from the anterior part, which is generally very much prolonged in 
the P. semireticulatus, and again the concentric wrinkles, which produce the 
reticulated appearance in P. semireticulatus, making it easily distinguishable 
from its congeners, are not well marked, and are scarcely noticeable in^P. 
magnus. 
Horizon and Localities. — This species was found in America, in the 
lower beds of the Carboniferous Limestone of County Monroe (Illinois), and 
in County Sainte- Genevieve (Missouri). Mr. W. B. Clarke found a single 
specimen of it in a brownish hrecciated limestone south-east of Buchan. 
Productus semireticulatus, Martin. 
PI. IX, Pig, 2.' 
ConcJice piles ce, 
Anomites semireticulatus, 
,, productus, 
Productus scoticus, 
„ Martini, 
D. Tire, 1793, Hist. Eutherglen, p. 31G, pi. 16, fig. 12. 
W. Martin, 1809, Petrif. Derbiensis, p. 7, pi. 31, fig, 1, 2, 3, 
and pi. 33, fig. 4. 
Idem, 1809, Hid., p. 9, pi. 22, fig. 1, 2, 3. 
Sowerby, 1814, Min. Conch., I, pi. 69, fig. 3. 
Idem, 1821, ibid., IV, p. 15, pi. 309, fig. 230. 
[Vide Etheridge, R. Junr., Geol. aud Pal. Q’laud, 1892, p. 265. — W.S.D.] 
