116 Prof. W. B. Benham — A Giant Fossil Cirrijjede. 



indicates the difficulty of drawing any hard and fast line between 

 the two genera. In fact, one of our New Zealand species, Scalpellum 

 villosum, is the very stumbling-block that he discusses. Nevertheless, 

 Darwin, from his great experience in identifying these Cirripedes, 

 indicates certain characters on which he relies, at any rate for fossil 

 forms, in discriminating between the two genera. 



In speaking of the carina, Darwin thus characterizes it for fossil 

 species of Scalpellum (p. 17) : it is "narrow, widening but little 

 from the apex downwards, slightly or considerably curved inwards " ; 

 and adds further down the same page that " the chief character 

 by which the valve can be recognised as belonging to Scalpellum is 

 the distinct separation, by an angle often surmounted by a ridge, 

 of the tectum from the parietes, which are either steeply inclined 

 or rectangularly inflected ; the lines of growth on the parietes are 

 oblique." 



On the other hand, in Pollicipes the carina is described (p. 49) 

 as being " either bowed inwards or is straight ; it widens from 

 apex downwards more rapidly than in Scalpellum ; generally a con- 

 siderable upper portion projects freely, and this portion is always 

 much less concave than the lower part." Further on he says, " the 

 parietes are generally more or less inflected, but they are not 

 separated by any defined ridge or angle from the tectum. Lines 

 of growth on the parietes are transverse or only slightly oblique." 



From these two quotations it would appear pretty certain that the 

 carina described above would be attributable to the genus Scalpellum. 

 But in examining the plates illustrating this genus we find in none 

 of the species a carina that resembles that under discussion. 



The present fossil has not only a much narrower tectum, but the 

 difference in width at the two extremities is very much less than in 

 any figured. The base, and therefore the lines of growth, are in the 

 latter angulated, instead of being transverse as in our fossil. Again, 

 the parietes are in most of the figures scarcely, if at all, visible in 

 a dorsal view of the valve, since they are much more steeply 

 inclined to the tectum than in the present instance. Further, the 

 parietes in Darwin's species of Scalpellum is either a continuously 

 curved plate of nearly equal width throughout, or its edge forms 

 a chord to the arc of a circle described by the tectum.' 



In our fossil we do not know with absolute certainty the true 

 curve nor the true length of the carina, nor the position of the 

 umbo ; nevertheless, the appearance presented by the oblique margin 

 of the distal moiety of the parietes leaves little doubt but that it is 

 the natural, uninjured margin. If this be the case, and if we suppose 

 this edge to be produced to meet a continuation of the tectum, we 

 shall obtain the true form and size of the whole valve, which is not 

 greatly curved nor much longer than the portion preserved, while 

 there is no reason to doubt that the umbo is terminal. 



Now it is a distinctly remarkable fact that the only carina figured 

 by Darwin that does bear resemblance in the points above referred 



1 There is one exception, Sc. magnum, in which, however, the umho is not 

 terminal as it is in the rest, and as it is, in all prohahility, in oiu- fossil. 



