GLABRAE. 89 



direction of the rows of costa are not conformable with the sulcations ; they are more 

 horizontal, thus rendering their aspect rather excentric ; upon the anterior face of the 

 shell they are entire, much attenuated, and form a slight angle or undulation ; the costae 

 upon the umbo form a very numerous, plain, minute, closely-arranged, horizontal series, 

 which pass also across the ante-carinal sulcation and area with slightly diminished 

 prominence ; subsequently the rows have their middle and posteal portions closely 

 arranged and very unequal in size ; they are usually slightly knotted or nodose, becoming 

 larger posteally, and terminate abruptly at the smooth ante-carinal space, but occasionally 

 over the lowest zone they are confusedly crowded, and minute or even continued across 

 the ante-carinal space. Our example, PI. XVIII, fig. 3, represents a well-marked 

 variety, with few prominent costse, each of which has about seven, large, widely separated 

 tubercles ; a specimen of more advanced growth has the last zone crowded with minute 

 tubercles, which afford a remarkable contrast to the other rows of costae, but the umbones 

 have their costae plain, minute, and dense, as in the typical form. 



Compared with T. gihhosa the general figure differs considerably. It is shorter 

 transversely, or more obliquely lengthened or ovate ; the concentric sulcations are smaller, 

 the umbones are more elevated, the costae are smaller, more numerous, and more minutely 

 nodose ; the very numerous hnear series which occupies the first zone of the shell and 

 passes across the whole of the valves transversely is also another remarkable and 

 distinctive feature. As there is much variability in the obliquity of the valves, measure- 

 ments of proportions would have but little utility. 



The first notice of T. Damoniana as distinguished from T. gihhosa occurs in a thin 

 quarto volume published by Miss Etherelda Benett, of Warminster, in the year 1831 ; 

 intended as an illustrated catalogue of organic remains in the Covmty of Wilts, it is 

 therein mentioned as a distinct variety of T. gihhosa ; the drawing is characteristic and 

 satisfactory, but the extreme scarceness of the work in question, together perhaps with 

 some supineness or absence of sufficient investigation by British palaeontologists, rendered 

 it altogether neglected as a species. 



StratigrapMcal position and Localities. T. Damoniana is an abundant fossil at 

 several localities, more especially in the white limestone of the Isle of Portland ; at Brill, 

 Bucks, and at Swindon, at the latter place the reddish ferruginous sandy beds at 

 Dayhouse Parm have produced very numerous separate valves of T. Damoniana, having 

 the ornamentation for the most part very well preserved ; the specimens are of every 

 stage of growth, but are frequently distorted by vertical pressure ; they are spread out 

 laterally and slightly flattened ; specimens are in the Museum of Practical Geology and 

 in the collection of Mr. Cunnington, of Devizes. Internal moulds are abundant at 

 Swindon. They are shorter and more oblique than those of T. gihhosa. 



A very large majority of examples of T. Damoniana obtained in the Isle of Portland 

 coincide with the foregoing description ; rarely, however, certain features become promi- 

 nent, indicative of an additional variety, characterised by the great breadth of the smooth 



