176 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



structure is so hard and compact and the sense-organs so fragile 

 that they are rubbed off very easily. If at any time the valves 

 are completely covered with an eye-pavement, then it is impossible 

 to calculate what numbers there are as they are so small, but I 

 have good reason for believing that they never do completely 

 cover the shell. 



Cytherea sp. 1 — The shell here referred to, has a highly enamelled 

 shining tegmentum, but is from Japan and not Australian. The 

 outer layer of shelly matter is of a rich brown colour and full of 

 small tubes which are abundantly supplied with nerves. Having 

 a section of this shell, I introduce it as an instance of a highly 

 enamelled shell with a brilliant polish on the tegmentum which 

 would seem incompatible with the sense-organs. Yet sense-organs 

 do exist even on this enamelled shell, and there is a large nerve- 

 ganglion, but I have not been able to detect the eyes as yet. 



Elenchus bellulus, Phil. — This shell belongs to a well-known 

 group distinguished by their richly iridescent nacre. The outer 

 plate of the shell is hard and compact, and sense-organs appear 

 to be present. 



Trochus (Calcar) tentoriiformis, Jonas. (Zeitschrif t fur Malak., 

 p. 66, 1845.) This remarkable shell is found abundantly in Port 

 Jackson underneath the ledges of rocks at low water spring tide. 

 The shape is very peculiar in one particular, that is the deep 

 concavity of the base whence the name of tent-shaped no doubt 

 has been derived. Mr. J. Brazier has drawn my attention to 

 another peculiarity, that is the many radiating grooves on the 

 lower whorls of the young shells, which make it resemble in some 

 respects Trochus imperialis of New Zealand, or T. costulatum y 

 Lam. In the adult species, however, the shell is so very much 

 corroded and encrusted, that these grooves disappear, and then 

 the spire is high and acute. The normal surface of the whorls is 

 covered with a series of very finely imbricated lines of growth, 

 much undulating and sometimes irregularly crossing or uniting with 

 each other. Under the microscope these lines appear to be studded 

 near the base with minute refractive lenses in great numbers, 

 occupying for the most part the hollows between the lines. The 

 shell is highly nacreous and apparently easily corroded. On the 

 base about half the surface is covered with finely divided spiral 

 lines or ribs equal to each other and regular. These are crossed 

 by very fine raised undulating shelly plates or imbricated lines, 

 grooved on the upper edge, and within this groove there are the 

 openings of what appear to be capsules and possibly sense-organs, 

 but of which a careful examination has not been made. At 

 pi. xii., fig. 19, there is a vertical section of the columella of this 

 shell, which in many respects seems to be one of the greatest 

 interest, but which as yet has only been partially examined. 



