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BRITISH FOSSIL TRIGONLE. 



carina, they enlarge rapidly downwards towards the pallial border ; the spaces between 

 the rows are plain and very wide. This well-marked variety has not been observed in 

 Britain. 



The fine specimen of T. dcedalea figured in the * Mineral Conchology,' now in the 

 British Museum, having so frequently been regarded as identical with the variety of 

 T. nodosa so abundant in the beds called "Crackers" of the Neocomian formation at 

 Atherfield, a comparison of the two forms becomes necessary. T. dcedalea has much 

 less convexity ; this feature alone will usually be sufficient to separate them ; it is also 

 shorter in the general figure, more especially upon the superior border, which has the 

 escutcheon small and inconspicuous. The area is more flattened and its ornamentation 

 is much more minute ; it is much less decidedly bipartite, or is without the concavity 

 formed by the upper half of the area in T nodosa. Its three rows of carinal nodes upon 

 the apica 1 portion of the valve are minute, very numerous, and closely arranged, so that 

 the transverse ridges which cross that portion of the area and are continued to the anteal 

 border of the valve are also greatly more numerous and closely arranged. The rows of 

 pallial varices have greater curvature, they pass upwards towards the angle of the 

 valve almost perpendicularly, or form a much greater angle with it than is seen in 

 T. nodosa ; the nodes in the rows are also more elevated and pointed ; they lessen in 

 size rapidly and regularly from the border upwards, so different from the irregular and 

 unequal nodes in T. nodosa, which, for the most part, have the largest nodes about the 

 middle of the rows. But apart from these details, a first glance at the depressed, 

 short figure of T. dcedalea will usually suffice to separate it from the Neocomian 

 species. 



The foregoing details will also serve sufficiently to separate the Devonshire T. dcedalea 

 from that different example of the Quadrates afforded by the Greensand of Le Mans, 

 with which, misled by Parkinson's very insufficient figure, Deshayes and D'Orbigny 

 united it. Sowerby (' Geol. Trans.,' 2nd series, vol. iv, pi. xvii, fig. 10) depicted a very 

 young, almost embryotic example of T. dcedalea under the name of T. quadrata. A 

 similar but larger specimen is given, Plate XXII, fig. 8, of the present Monograph ; it 

 should be compared with the young example of T. nodosa, Plate XXIV, fig. 3. Four 

 years subsequently to the appearance of Sowerby's figure, Agassiz, who had received 

 specimens of T. dcedalea from England, was therefore fully aware of their distinctness 

 from the large species of Le Mans, figured and described the latter under the name of 

 T. quadrata (' Trigonies,' p. 27, tab vi, figs. 7 — 9) ; apparently he was unaware that 

 Sowerby had appropriated that name for his little Greensand specimen. By reuniting 

 the little species of Sowerby to T. dcedalea the name quadrata given by Agassiz to the 

 large species of Le Mans will thus be entitled to remain ; it has not been obtained in 

 Britain. Possessing such materials for comparison it is remarkable that Agassiz, upon 

 page 26 of the same work, should have tabulated T. dcedalea, Park., as identical with a 

 Trigonia from the Portland formation of Besancon, which he figured and described 



