4 FOSSIL CIRRIPEDIA. 
resemblance in mere outline. The peculiar cancellated structure, which is almost visible on 
the external surface even to the naked eye, is wholly unlike anything known amongst 
Cirripedia ; a thin polished slice of the valves of Lepas and of Aptychus, viewed under a high 
power, are as unlike as anything can well be.' In Aptychus the lines of growth are 
conspicuous on the inner or concave surface, and indistinguishable or not plain on the 
outer surface; whereas in Lepas exactly the reverse holds good. Again, in some 
specimens it appears, that additions are made to the shell on the exterior edge of the 
growing margin, instead of on the inner edge, as in Cirripedia. In Aptychus latus, 
there is a rather deep internal fold along the whole of that margin, through which the 
cirri are supposed. to have been protruded, and this is unlike anything which I have met 
with in Cirripedes. In all the species of Aptychus, the two valves are much the most 
frequently, though not invariably, found widely opened, and attached together, either 
exactly or nearly so, by the two margins through which the cirri must have been protruded. 
Now in all true fossil pedunculated Cirripedes, the valves are found either separate, which 
is the commonest case, or when held together, those on the opposite sides almost exactly 
cover each other, for there is nothing in the structure of Cirripedia tending to open the 
valves like the ligament in bivalve shells. How comes it, then, that the specimens of 
Aptychus, even those found within the protected chambers of Ammonites, thus generally 
have their valves widely gaping? Even if we pass over this difficulty, is it not strange 
that the valves should always have been held together by that margin, which in the recent 
condition is supposed to have been open for a considerable portion of its length, for the 
exsertion of the cirri; whereas, in not one single instance, as far as I have seen, are the 
two valves held together by the opposite margin, which in the recent state, on the 
idea of Aptychus having been a Cirripede, must have been continuously united by 
membrane. 
There is another argument against Aptychus having been a Curipede, which will have 
weight, perhaps, with only a few persons: in Pollicipes, the main growth of all the valves 
is downwards ; in Lepas or Anatifa, as well as in most of the allied genera, the main 
growth of the Scuta and of the Carina (7. e. lower lateral, and dorsal, or valves,) is in a 
directly reversed direction, or upwards. Now Pollicipes is the oldest known genus of 
Cirripedes, having been found in the Lower Oolite, whereas hitherto Lepas is not 
certainly known to have been discovered even in the newest Tertiary formation. So again 
within the limits of the genus Scalpellum, I know of only two cretaceous species in which 
the Scuta grow upwards and downwards, and only one case in which the Carina has this 
double direction of growth ; whereas in the recent and one Miocene species, these valves 
usually grow both upwards and downwards. Hence it would appear that there is some 
relation between the age of fossil Lepadidz and the upward or downward direction of 
1 When I had the slices made, I did not know of H. von Meyer’s paper on Aptychus, in the ‘Acta 
Acad. Cees. Leop. Car.,’ vol. xv, Oct. 1829, tab. lviii and lix, fig. 13, in which perfectly accurate sections 
are given of the microscopical structure of Aptychus levis. 

e. 
