FISTULANA. 



covered with an horny epidermis^ have the means of per- 

 forating Madrepores and other calcareous substances; 

 they do not, however, themselves make any calcareous 

 lining to the tube they form, so that the small stellated 

 tubes of the Madrepores open into them on all sides ; 

 when the animals of the Lithodomi die, the shell remains 

 in the perforations, which, together w^ith the shells, be- 

 come filled up with all sorts of extraneous substances 

 which fall into them and also partly occupy the small 

 stellated tubes of the Madrepore : afterwards, when the 

 whole has become fossilized, a part of the Madrepore it- 

 self, being of a difi'erent substance from that filling the 

 cavities, and more easily acted upon by external circum- 

 stances, frequently becomes decomposed and is carried 

 away, leaving the substance which formerly filled the ca- 

 vities, originally formed by the perforation of the Litho- 

 domi, exposed in the form of a date-stone, or some more 

 clavate body, attached to the mass by the smaller extre- 

 mity, rounded at the larger, containing the two valves of 

 the Lithodomus, and covered on the outside with small 

 irregular stellated prominences, which are those portions 

 of the substances formerly deposited in the cavities, that 

 had fallen into the stellated tubes of the Madrepore. In 

 this case the substance surrounding the shell has been re- 

 garded as the tube which contained it, and it has conse- 

 quently been referred to the Genus Fistidana, as belong- 

 ing to the Tubicolce, and having two free valves inclosed 

 in a shelly tube. As we have now fairly excluded all La- 

 marck's FistulancE except his F. Clava^ we are disposed to 

 think that this ought to be considered as a distinct Genus, 

 for the following reasons ; it has a regular straight clavate 

 tube, the two free valves that are included in the lower 

 and larger closed extremity are very inequilateral and of 

 a transversely elongated form, its animal does not perfo- 

 rate any substance, but merely forms its tube in the sand, 

 particles of which frequently adhere to it, and when com- 

 plete the two valves are confined to the lower part of the 

 tube by a septum, open in the centre, and placed in the 

 tube near their posterior and superior extremity : in all 

 these characters it differs from GastrochcEna, to which it is 

 most nearly allied, and with which it is associated by 

 Spengler. The Gastrochaena Lagenula (Lamarck's Fistu- 

 lana Lagenula) a^ipesirs to connect it very closely with the 



