302 
SUPPLEMENT TO THE BRITISH 
" It is only when the valves are decorticated that the orifices of the perforations 
exhibit themselves. Still, supposing that the caeca are in some way or other concerned 
in the function of respiration, I suggested that the epidermis, which I showed in my 
Memoir to be cellular, may be so far pervious as to permit external water to pass through 
it to the filaments and the csecal appendages. 
" Whether or not this idea is correct I must leave for others to determine. Nor do 
the observations of Albany Hancock tend to settle the question, for, although it is his 
opinion that the caecal appendages can scarcely have anything to do with respiration, as 
some have supposed, he admits that the spherical bodies which he detected in them may 
be blood-corpuscles.^ 
"Returning to the perforations. It is mentioned in my Memoir that Crania anomala 
and Terebratulina caput-serpentis exhibit a curious modification of their extremities. The 
perforations are wide at their base or on the inner surface of the valves (in which respect 
there is some resemblance to the tubulation of Productus), and instead of abruptly ending 
in disc-shaped orifices, surrounded by a fringe of tubuli, as ordinarily, they become 
attenuated and divided (see figs. 10, 11, 13, 16, and 18 of my Memoir), their termina- 
tions assuming an arborescent appearance ; but even in this form the perforations do not 
present any certain appearance of ending by penetrating the epidermal layers. 
" Now, as this character seems to be general to all the perforated Palliobranchs, it 
strikes me that there is nothing anomalous in the perforations of Productus ; accordingly 
I would suggest that in the perforated species of this genus the valves were covered with 
a more or less calcified epidermis, and that, like its corneous homologues in recent 
Palliobranchs, it closed the orifices of the perforations. It is probable, however, that the 
cEecal appendages, undoubtedly contained in them, held communication with the external 
water by means of capillary pores in the epidermis. 
" I dare not offer any suggestion as to whether there is any connection between the 
perforations and the spines which cover the valves of the Productus as mentioned in the 
beginning. The spines are of different sizes in P. punctatus, the smallest being densely 
crowded ; but they are so attached to the epidermal layer that the latter, exceedingly 
thin, ' peels off,' leaving only its inner surface exposed, and consequently a portion of its 
outer surface, with cicatrices of spines, is visible. It is curious that the inner surface of 
both valves, particularly the flat one, is well furnished with mammillar pustules. In a 
specimen before me, however, of P. semireticulatus the processes have taken the shape of 
pointed spines." 
1 'Trans. Roy. Soc.,' vol. cxlviii, pi. Iviii, figs. 5 and 8, pp. 827—836, 837, 1858. 
