GLABRA. 87 



indicated near to the umbones by a well-marked obtuse ridge, but this soon disappears 

 with advance of growth, and the carina is then represented only by the rounded elevation 

 which the border of the area forms, adjoining to the wide and very depressed smooth or 

 ante-carinal space ; the plications of growth are well defined over the whole of the area. 

 The other portion of the surface has a very numerous and well-marked series of obliquely 

 directed tuberculated costse ; upon the umbones the costse are different ; they there form 

 a densely-arranged minute or linear series which pass horizontally across the whole of 

 the valve uninterruptedly ; they are plain and the breadth of the series does not exceed 

 three Hues (this feature, unfortunately, is not depicted upon our figures 4 a and 4 b) ; the 

 costse then change abruptly to oblique tuberculated rows which continue with only slight 

 irregularity even to the lower border ; the costse (about twenty-four in number) are 

 narrow, closely arranged, curved, and somewhat attenuated near to the pallial border ; 

 they pass upwards in a manner sometimes somewhat waved and meet the depressed 

 ante-carinal space at a considerable angle, the tubercles in the rows (about twenty) are 

 rounded or ovate and closely arranged, but upon the anteal attenuated portions of the 

 costae they are indistinct or cord-like ; the costal terminations, posteally, are abrupt but 

 do not form a regular line, so that the anteal boundary of the ante-carinal space is 

 irregular. The arrangement of the rows is so close that it is sometimes difficult to 

 discover the real direction of the lines of tubercles ; in such instances the attenuated 

 pallial extremities of the rows of costse afford the real guide. The ante-carinal space is 

 less wide than in T. Miclieloti and T. gihbosa, but it is always conspicuously depressed 

 longitudinally, which imparts an additional apparent convexity to the area. The valve 

 has three transverse sulcations or arrests of growth ; these are not very conspicuous and 

 do not appear materially to have interfered with the direction of the rows of costse which 

 pass across them. 



This pretty species constitutes one of the most clearly defined examples of the Glabra, 

 the direction and arrangements of the rows of tubercles immediately suffice to determine 

 the species ; perhaps it may be the shell of which a fragment of the anteal side was 

 figured by Agassiz, ' Trigonies,' tab. 6, fig. 11, under the name of T.picta, from the White 

 Coral Crag of Hoggerwald (Canton of Soleure), the general direction of the hues of 

 tubercles and their attenuation towards the border agree with T. Manseli, but in the 

 absence of the greater and more important portion of the valve I prefer only to allude to 

 their possible specific identity. 



Upon the whole there is much variability in the characters of the tuberculated costse ; 

 occasionally the anteal portions of the umbonal rows are somewhat angulated or are 

 curved concentrically, or the anteal portions of the latter-formed series have an occasional 

 intercalated costa. The varying size of the tubercles in different specimens, and their 

 more close or distant arrangement in the rows, impart much variability to the aspect 

 of the species, but in every instance the tubercles are rounded and have much 

 prominence. 



