Mr. W. Clark on the Pholadidae. 335 



in several of the ovaria the contents had begun to assume the 

 appearance of ova, but with the utmost power of the microscope 

 I could not perceive any trace of the membz*anous pyriform 

 bodies containing a fluid which I have observed in the genial 

 months in many of the ovaries of the strict hermaphrodite bi- 

 valves and Gasteropoda, which I consider to be the male influ- 

 ences ; nor could I discover any other organ that had the slightest 

 pretension to be regarded of a similar nature. I failed to verify 

 the oviduct, but from the position of the ovarium I have little 

 doubt that it passes at and under that portion of the peritoneal 

 cavity which contains the terminus of the ovary, into the dual 

 compartment of the mantellar tube, about \^ inch above the 

 siphon, and that the ova are there discharged. I do not think 

 the branchiae in this species serve as receptacles for the ova ; their 

 structure is not calculated for such purpose. 



I have now to make a few observations on the camerated struc- 

 ture of the posterior part of the protective tube, which has caused 

 malacologists much speculation on the uses of it ; some, myself 

 amongst them, thought the laminar spaces might be for a time 

 to protect the puUi until ultimate exclusion. 



These ideas were dispelled by the discovery of the fixture of 

 the posterior part of the animal by the strong oval muscles 

 springing from the sphincter, which induced me to examine this 

 portion of the tube with care, and in a fine full-grown specimen 

 I discovered the principal object, if not the precise animal oeco- 

 nomy, of the lamina. I perceived in the centre of each plate a 

 decided muscular impression, which, on comparison with the 

 last-formed one of the sphincter muscle, proved to be identical 

 in shape ; this fact made it evident, that the animal, either when 

 full-grown, or when growing, if its longitudinal increase is not 

 correspondent with the boring progress, must, by being poste- 

 riorly fixed, either suspend excavation, rupture the mantle, or 

 have the power of advancing the muscle of attachment. This ad- 

 vancement of the muscle is not a new fact ; it has been observed 

 in the Spondyli and Ostrea; and it cannot be doubted that na- 

 ture has conferred on the present species the power of detaching 

 and advancing the muscle of attachment, and that each hoop- 

 shaped lamina, thrown out for some point of the animal oeconomy, 

 marks the periodic removal of the muscle. The laminse are 

 always more numerous in the longer and older animals ; in 

 very young specimens there are only 1-3, and in the older 

 ones 20-40. 



When authors have stated that this species has the tube with- 

 out concamei'ations, we presume they have only had opportunities 

 of examining very young or imperfect specimens ; in all the spe- 

 cimens I have seen, many of which were 10 inches long, they 



