(JO ZOOLOGY. 



is soon after able to commence perforating. The transparent shell of the 

 embryo soon disappears, and is replaced by the permanent one. In boring, 

 the wood is generali}^ entered across the grain, but afterwards the animals 

 turn in the direction of the fibres, though they are subject to be continually 

 turned from their course by each other, the wood being perfectly honey- 

 combed by their number, and often in the course of a year. 



Fam. 8.* Tiibicola. The name indicates the tubular form of the external 

 protection in this family. The tube is secreted by the mantle, and its 

 continuation covering the siphons. In Aspergillum {pi. To, fig. 71) both 

 valves form part of the tube ; in Clavagella the valves are placed within a 

 short tube, one of them being attached ; and in Gastrochcena the shell is 

 not attached, but inclosed in a tube. The mantle is open in front for the 

 ])assage of a little foot, the siphon and branchiae are usually long, and the 

 palpi are small. These shell-fish inhabit clay, sand, or cavities in stone or 

 wood, pierced by themselves. 



The genus Aspergillum was first correctly figured in 1685, by Lister, one 

 of the fathers of conchology ; but it was many years before its true 

 approximate place was known, since it required much sagacity to detect a 

 bivalve shell under the form of a tube. We accordingly find it placed at 

 one time among univalve genera, and at another with the Annelida which 

 form tubes, and Linngeus placed it with Dentalium. The happy idea of 

 Aspei-gillum being a true bivalve is due to De Roissy, and is a remarkable 

 example of the detection of natural aflinities. A parallel example appears 

 in the Planürhis cornuarietis which, to all appearance, was referred to its 

 pro]>er genus, but which Sowerby referred to Ampullaria, with none of 

 . which did it seem to be allied ; but the subsequent discovery of the animal 

 proved his reasoning to be correct, although strongly opposed at the time. 

 The structure of Clavagella is quite anomalous, the valve of the right side 

 forming part of the inside of the tube, and the left valve being free. The 

 anatomy is essentially that of a bivalve mollusc, and shows an aflinity with 

 Aspergillum. It is provided with a small foot. 



Tlie genus GastrocJitena, Spengler, 1783 (or Fiskdana, Lam., 1801), is a 

 boring shell, sometimes found in calcareous rocks, and sometimes in sand, 

 Hucrethig, besides its proper shell, a separate exterior tube ; and, as the shell 

 may be accidentally found without the tube, after the death of the animal, 

 jin error was committed in describing the shell alone as one genus, and the 

 tube and shell together as another, namely, Fistulana; an error which 

 Deshayes was the first to correct, and he accordingly suppresses the latter 

 name. Shell symmetrical, and widely gaping anteriorly, without cardinal 

 teeth, a ligament posteriorly, pallial impression deeply sinuated, foot small, 

 and secreting a byssus. The following species are found in the tertiary 

 strata of the United States. Gastrochrma Ugula., II. C. Lea, Am. Phil. 

 Trails.^ vol. ix. G. larva, Conrad {Fistulana), Am. J. S&L, 1846, p. 213. 

 G. dongata, Desh., Conrad, J^at. Inst. WasUngton. 



Ti'ihe Elaiohranchia. 



This tribe includes the great mass of Conchifera, after separating the 

 preceding one. With very few exceptions, the Inclusa have two adductor 

 264 



