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BRITISH FOSSIL TRIGONLE. 



the valves have the cardinal teeth much smaller and less prominent than in T. nodosa 

 and T. spectabilis ; the pallial posteal pits and elevations are small, closely arranged, 

 and only three in number, thus affording a contrast to the same feature in T. nodosa, 

 which has them much larger and greatly more numerous, so that in that species they 

 occupy much of the surface near to the pallial border. 



The foregoing detailed description will, it is trusted, suffice to rectify an error into 

 which some palEeontologists have fallen, who have taken for their guide the figure of 

 T. dcedalea in the ' Mineral Conchology' of Sowerby, an inaccuracy in the drawing 

 having produced much confusion from its supposed identity with that fine variety of 

 T. nodosa of which such considerable numbers have been obtained in the beds called 

 " Crackers " in the Neocomian formation at Atherfield. Sowerby's drawing of 

 T. dcedalea has the angle of the valve occupied by a row of rounded nodes, which are 

 larger than those in the adjacent rows of varices and have no accordance with them 

 either in number or position. This arrangement differs materially from the original 

 specimen now in the British Museum, but has much resemblance to the corresponding 

 parts in T. nodosa. As the figure of T. dcedalea in the ' Mineral Conchology ' represents 

 the only example of adult growth to be found in any British work since the 

 fragment figured by Parkinson, our figures are the more deserving of scrutiny and 

 comparison. 



The variability so frequent in the ornamentation of the surface in the typical form 

 may now be adverted to. Occasionally the minute tubercles upon the area are arranged 

 with regularity, forming oblique rows which curve upwards and backwards from the 

 angle of the valve. Another arrangement has the row of small nodes upon the angle 

 of the valve, assuming the same comma-like figure which is so conspicuous in the 

 larger carinal nodes of T. nodosa ; how T ever, this feature is scarcely ever distinct upon the 

 whole of the row. Near to the pallial border, occasionally, are one or several short 

 supplementary rows of varices resembling the same feature which is common in 

 T. nodosa. The internal mould appears to be unknown. 



A larger and well-marked variety of T. dcedalea, var. confusa, is illustrated by 

 Plate XXIII, fig. 1, which represents a specimen obtained by Mr. Vicary, of Exeter, 

 in the Greensand of Little Haldon ; it is distinguished by the generally increased size 

 of the several features which ornament the surface, by the larger pallial varices and 

 their nodes, and more especially by the unusually large, confusedly irregular, and 

 unequal nodes scattered over the area ; there is also a series of short anteal supple- 

 mentary pallial varices, several of which are intercalated with the anteal extremities of 

 the large oblique varices. This variety is rare, no second specimen has come under my 

 notice. 



Measurements of two examples of the typical form in my collection : 



