31 



quorum anteriora maxima sunt et inter se oontinuatione augusta juncta. 

 Species hujus generis iionnuUis speciebus generis Metoptomas secun- 

 dum desoriptiones et figuras a Billings in libro ' Canadian Organic 

 Eemains' " (sic) vol. 1, page SY cet., datas affines sunt. Generi JSTacellje 

 Schum. e tribu Patellidarum quoad formam valde congrucns, hocce 

 genus silui-icum impressiones musculorum ad instar Olanas (Patellae) 

 cochlearis L. dispositas habet." Lindstrom. 



As already partially suggested by Mr. Dall,* it seems clear that of 

 the nineteen species provisionally referred by B. Billings to Phillips' 

 genus Metoptoma, in the first volume of the "Palaeozic Fossils" of 

 Canada, not one of them really belong to that genus as now under- 

 stood. As the name implies, in Metoptoma proper the widest end of 

 the basal margin, which was supposed by Prof. Phillips to be the 

 anterior end, is distinctly concave or notched, a character which is not 

 possessed by any of the so-called species of Metoptoma from the Cam- 

 bro-Silurian or Silurian rocks. In the writer's judgment Metoptoma 

 Quebecensis, Billings, belongs to the genus Palmacmosa of Hall and 

 "Whitfield : M. Niobe, M. Nycteis, M. Eubule, M. Erato, and M. Hyrie, 

 Billings, are typical species of Tryblidium, Lindstrom : M. Trentonensis, 

 M. Estella, M. instabilis and M. simplex, Billings, appear to differ only 

 from the generic characters of Tryblidium in that the outline of their 

 basal margins is sub-circular rather than ovate : while the rest of the 

 species described by Billings in the volume cited are probably types of 

 two or three new and at present uncharacterized genera. 



The Guelph Formation has yielded a single specimen of an interest- 

 ing new species of Tryblidium, which may be thus described. 



Tryblidium Canadense. (N. Sp.) 

 Plate 5, figs. 1 and la. 



Shell patelliform, conical, much depressed: highest a little behind 

 the middle (as viewed laterally), sloping rather abruptly downwards 

 behind the most prominent point, and more gradually towards and 

 down to the apex in front : sides obliquely convex : apes placed very 

 near to the anterior end, but not quite terminal, pointed, incurved, 

 but scarcely hooked, and depressed below the greatest elevation to a 

 distance of rather more than one-half the entire height : base broadly 

 ovate, narrowest under and in front of the apex : length greater than the 

 breadth : maximum height less than half the bi-eadth : muscular im- 

 pressions not satisfactorily shown : surface marking unknown. 



* In the American Journal of Conchology, vol. 6, p. 281. 1881. 



