POSIDONIELLA KIRKMANI. 97 



Dimensions. — Fig. 16, PI. VI, measures — 



Antero-posteriorly , . . .9 mm. 



Dorso-ventrally . . . 9 mm. 



Elevation of valve .... 2'5 mm. 



Observations. — By the courtesy of Mr. Hoyle and Mr. H. Bolton, Manchester 

 Museum, Owens College, I have been able to study the series of shells from 

 Todmorden named and figured by Captain Brown. In addition to these, several 

 specimens were found with MS. names; but I am of opinion that, with one or two 

 exceptions, these were only varying forms of those previously named, due to the 

 state of preservation or growth. 



The distinctive character of this species is the non-terminal position of the 

 umbones, and the originals of the three published species of Brown's Gatillus 

 Kirhnani, G. costatus, and G. obliquatus have this peculiarity in common, although 

 there may be some uncertainty (owing to the lapse of time, and the fact that the 

 labels were loose) as to which was the original of each form. Still I believe, on 

 comparing the shells with Captain Brown's figures, that fig. 16, PI. V, in this 

 Monograph represents G. Kirhnani, fig. 20 0. costatus, and fig. 19 C. obliquatus; 

 the presence or absence of the anterior ear depending on the proper clearance of 

 matrix in front. The specific name " Kirhnani " is retained, as it came first in 

 Captain Brown's paper (op. cit.). 



I would, however, draw attention to the fact that Captain Brown's figures are 

 all much larger than the shells now illustrated. 



The original description of P. Kirhnani is — " Oblong-ovate, convex, smooth, 

 sides nearly equal, marked with four concentric lines of growth. Its umbones 

 prominent, central, and rounded ; with several inequidistant concentric lines of 

 growth ; margins sharp and even." The number of prominent lines of growth 

 depends, of course, on the age of the shell and its environment ; and more 

 especially on the condition of the fossil, whether it is semi-decorticated or not. I 

 would point out that Brown's figures of G. obliquatus and C. costatus show several 

 equidistant lines of growth ; but in these cases larger shells were depicted than 

 that of G. Kirhmani. In the description of G. obliquatus the umbones are stated 

 to be " placed considerably to one side," but the figure shows them to be sub- 

 central, the slight difference in the two specimens being due to their different ages 

 and sizes. A similar tendency to invent species on the weakest evidence, the 

 result of accuracy in ocular observation only, is also shown in Brown's work on 

 the " Unios (Carbonicolas) of the Coal-measures," in the ' Annals Mag. Nat. Hist.,' 

 vol. xii, 1843, where twenty-six new species are described and figured, every 

 slight variation in form earning a new specific title. 



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