270 MR. R. OWEN ON THE ANATOMY OF CLAVAGELLA. 
remains free, or is connected only to the soft parts and cardinal ligament, in order to 
assist in the excavating and respiratory actions. 
That these actions are of a powerful kind is to be inferred from the remarkable de- 
velopment of the muscular system in the Clavagella. ‘The impression of the great or 
posterior adductor! is carried 2 lines beneath the surface of the chamber posteriorly, 
but gradually rises to the level of the valve. The impression of the smaller anterior 
adductor? is fainter, and is continued into the sinuous pallial impression, which fol- 
lows the contour of the anterior margin of the valve at about 2 lines’ distance from it. 
In the free valve* the last two muscular impressions are separate. 
The shelly substance of the fixed valve passes without interruption into that of the 
tube: a slight ridge circumscribing the entry of the tube into the chamber may be re- 
garded as the line of separation; unless the extent of the valve be limited to that of 
the internal nacreous deposition. 
The area of the tube is of an oval form, in diameter 7 lines by 5. The calcareous 
parietes are ;!;th of an inch in thickness at the outlet of the tube, and about 35th at 
the opposite extremity. As far as it is preserved in the present specimen no percep- 
tible increase is recognizable as it approximates the chamber. 
The free valve is an unequal triangle, with the angles rounded off, about the thick- 
ness of a sixpence, moderately concave towards the soft parts, striated only in the di- 
rection of the layers of increment on the outer surface, as in most of the Pyloridean 
Bivalves of M. de Blainville. The layers of increment of the free valve gradually in- 
crease towards the dorsal edge for a little more than one half of the valve, beyond which 
the layers continue of almost equal breadth. This growth of the valve corresponds to 
the direction in which the chamber is enlarged, which is principally on the dorsal, dex- 
tral, and anterior sides: now this is the mode of enlargement best adapted for the full 
development of the ovary; so that it would seem that the Clavagella continues for a 
certain time to work its way into the rock without material increase of size, leaving 
behind it a calcareous tube, which marks its track ; after which it becomes stationary, 
and limits its operations to enlarging its chamber to the extent necessary for the ac- 
complishment of the great object of its existence. 
The mantle envelopes the body like a shut sac, but is perforated, as before mentioned, 
for the siphon and foot, the opening for the latter part being reduced to a small slit?. 
An analogous orifice was observed by M. Riippell in the corresponding part of the 
mantle of Aspergillum, viz. that which is next the sunken sieve-like extremity of the 
tube, and by which he supposes the water necessary for respiration to be received when 
the retreating tide leaves exposed the expanded siphonic extremity. 
This cannot, however, be its use in such species of Clavagella as reside, like the 
present, at depths too great to allow of their being ever left with the siphonic aperture 
1 f’. Figg. 8, 10. 29’. Figg. 8, 10. 3h’, Figg. 8, 10. 
* Figg. 8, 10. 5 *, Figg. 12, 13, 14. 
