178 



erect sheath without let or hindrance, and when it reaches the surface, the 

 edge, as in Clavagella, becomes elegantly furbelowed. A season of rest 

 ensues ; another effort is made to extend the sheath, the growth is pushed 

 on, but the calcifying energies of the mollusk either are not needed, or are 

 enfeebled. A little is added to the sheath, and the edge is again furbe- 

 lowed, and in some specimens as many as eight times this process has beeD 

 repeated. In adherent species, such as A. Strangei, one specimen of which 

 was found affixed to the inner cleft of a mussel-hinge and the other to a 

 stone, the disk is completely smashed in, as it were, and the sheath being 

 free, not pushed through sand or any debris of shells or pebbles, is smooth 

 and tortuous, faintly coloured by an enveloping periostracum. I incline 

 to dissent from Dr. Gray's views that the sheath of the adhering A. Strangei 

 is an enlargement of the primitive pair of valves, while in the non-adhering 

 species it is analogous to the tubular sheath of the Teredo. The sheath of 

 Aapergillum has, as in all the Tubicola, a specific limit and mode of growth, 

 but whether by a stretch of philosophical induction it be regarded as an 

 enlargement of the primitive pair of valves or not, the relation between the 

 valves and the sheath I hold to be the same, whether living buried in sand 

 or adhering in an exposed manner to shell or stone. Dr. Gray also draws 

 a distinction between species which have a wavy depression round the pair 

 of valves, and those which have not, regarding the wavy depression as a 

 part of the valves of which only the umboes are exposed. My own view 

 is, that, at the time of the metamorphosis of the mollusk, the valves are 

 not larger in any species than are defined by the smaller outline. When 

 it is considered that the valves are cast at this time, but not entirely, in- 

 asmuch as they are appropriated as material for a nucleus from which to 

 develope a sheath, it is only reasonable to suppose that the new sheath- 

 matter would, for a time, obtain a wavy deposit corresponding with the 

 outline of the nucleus. 



Nineteen species of Aspergillnm are described and figured in my mono- 

 graph of the genus in ' Conch. Iconica/ but it is more than probable that 

 they may be reduced by further researches to about a dozen. They have 

 been named in Mr. Cuming's collection by Dr. Chenu and M. Deshayes, 

 and I have preferred to give them as they left them, except in a few in- 

 stances, where the identity with others was too obvious to be overlooked. 

 More specimens of the seven or eight doubtful species are needed before 

 rejecting them' as synonyms, or discussing their characters with anything 

 like critical accuracy. There are six well-marked types of Aspergillnm : — 

 1. A. vagimferum {Warnea, Gray), in which the sheath is furbelowed at 

 the top, and has a well-fringed disk at the bottom, with a wavy depression 

 around the valves. 2. A. Cumingianum, in which the sheath has a clumsy 

 distorted growth peculiar to the species, and is closed at the bottom by a 

 rudely convex disk, perforated by a bunch of tubes, not radiating in a frill ; 



