GRAMMOCERAS TOARCENSE. 171 



is uncarinate but is becoming sharper. The keel may be said to commence before 

 three lines of diameter are attained ; and at four and a half lines there is a well- 

 defined carina bordered by two small depressions. As yet the specimen is smooth, 

 for the ribs are not visible until a diameter of about five and a half lines. 



It is not without considerable hesitation that I have retained a name for this 

 form as well as for Gramm. striatidmn. Both Oppel and Dumortier have done so ; 

 but they have based their distinction upon the observation that in the former the 

 ribs run across the side to the inner area, while in the latter they do not. This 

 definition, however, though correct in the main, does not, I find, always hold good ;^ 

 and it is, besides, somewhat uncertain. The form to which I apply the name 

 toarcense is that which is lower in the scale than Gramm. striatulum, that is, it has 

 a squarer, more flattened ventral area, and generally possesses rudimentary sulci 

 at a certain age ; it has a thicker, more quadrate whorl with coarse well-marked ribs, 

 which generally, but not always, run right across the whorl; it has a more open 

 umbilicus generally. I use the word " generally " because the form is so variable 

 and does undoubtedly shade into Gramm. striatulum ; but I prefer to keep 

 the name toarcense to mark this particular stage of development. I cannot call 

 toarcense a variety of Gramm. striatulum, because it is not. If anything, the 

 relationship of the two forms is correctly described as Gramm. toarcense mutating 

 into striatulum — the latter being in all probability only a further development of 

 the senile stage of the former. At the same time we must remember that it may 

 possibly not be so; because both may be descended from a common ancestor 

 — Gramm. striatulum being the more changed form. In this case our present 

 nomenclature would be correct. Again, we must remember that Sowerby's name 

 has priority over d'Orbigny's, and that we have no right to place it in subjection 

 thereto. On all these grounds I prefer to keep the two names. 



In Gramm. toarcense the rudimentary sulci on a square ventral area which 

 generally appertain to the middle-aged specimens are the most important features 

 in separating the two forms ; because this is a stage which Gramm. striatulum does 

 not exhibit. At all ages the latter possesses an acute ventral area. 



The well-marked specimen of Gramm. toarcense shown in PL XXVIII, figs. 

 4, 5, is easily separable from Gramm. striatulum ; but the form to which d'Orbigny 

 gave his name is not so pronounced as this very coarsely-ornamented example. 

 A specimen which I have had depicted in PI. XXVIII, figs. 14, 15, is intermediate ; 

 because, while it possesses the acute ventral area of Gramm. striatulum, it shows 

 the coarse ribs and pronounced inner margin of Gramm. toarcense ; while its more 

 numerous irregular ribs, some of which disappear on the inner area, are a feature 

 peculiar to itself. 



Another form is still more truly intermediate. It possesses the shape — the 

 ' See PI. XXVIII, in which the contrary is exhibited. 



