298 Canadian Record of Science. 



Sowerby determined as P. Knightii are really referable to 

 the present species. However this may be, it seems to the 

 writer that P. decussatus differs materially from the true 

 P. Knightii, especially in the following particulars. The 

 umbo of the ventral valve of the former is narrower and 

 less prominent, while its beak is much less strongly curved ; 

 the coarser surface markings of both valves do not consist 

 of comparatively distant and regular radiating ribs, as in 

 P. Knightii, but of close set, irregularly disposed, unequal 

 and not much elevated radiating raised lines ; and the 

 mesial septa of both valves of P. decussatus are not more 

 than half the comparative length of those of P. Knightii. 



Cephalopoda. 



Gomx>hoceras parvulum. (Sp. nov.) 



Plate iii, figs- 5, 5 a, b. 



Shell small, straight, slender, rather more than three 

 times as long as broad, and broadest a little in advance of 

 the midlength : sides slightly compressed, the outline of a 

 transverse section near and at the commencement of the 

 body chamber being ovate : venter narrower than the dor- 

 sum and especially so at both ends : lateral outline conical, 

 with the ventral border not much more convex than the 

 dorsal. Septate portion occupying a little more than one- 

 half the entire length, narrowly conical in lateral aspect, 

 pointed posteriorly and about twice as long as it is broad 

 anteriorly. Body chamber crenulated around the base, its 

 outer margins at first nearly straight and almost parallel 

 on both sides as viewed laterally, its anterior termination 

 rounded but much more broadly so on the ventral side than 

 on the dorsal: ventral region at the summit laterally com- 

 pressed on each side of the aperture. Aperture, as viewed 

 from above, extremely contracted, Y shaped, with the stem 

 about twice as long as either of the two branches, which 

 diverge from it at an angle of about 115°. The stem is a 



