JOUSSEAUMIA. 261 



non-plicate gills are common among other sub-orders of Enlamellibranchia. The 

 detailed study of gill structure has therefore proved disappointing for classificatory 

 purposes, and in trying to trace the connections between the sub-groups of the 

 Enlamellibranchia we are once more thrown back on a criticism of the ensemble of 

 the anatomical characters of each family. The difficulty of placing any given genus 

 in its proper position in the system is well illustrated by Jousseaumia.. Its heterodont 

 hinge, sub-equal adductor muscles, entire pallial line, single pallial suture and very 

 simple gill structure leave no doubt that it must be placed in the Submytilacea, but 

 when one looks for its nearest allies in this very heterogeneous sub-order, the 

 difficulties are considerable, and they are not lessened by the fact that some 

 systematists give a certain character as diagnostic of a family, and then proceed to 

 describe as members of that family genera in which this diagnostic character is 

 wanting. 



Thus, Jousseaumia shows undoubted affinities to the Erycinidse (Leptonidse, 

 Pelseneer, 12). The members of this family are hermaphrodite, the ligament is 

 internal, the foot linguifbrm, elongated and byssiferous, the gills simple and 

 astartiform, with very little sub-filamentar tissue and scattered interlamellar junctions. 

 Many members of the family are of minute size and some (Lepton) are commensal. 

 Bouvier identified Jousseaumia as a Kellia, and E. A. Smith identified it with 

 Angas' genus Mysella, which Fischer (6) regards as closely allied with Kellia. 

 I have shown that it cannot be placed in either of these genera, and though it 

 might be regarded as having affinities with Lepton or Lascea, because of the single 

 pallial suture, it differs from the whole of the Erycinidse in the absence of the 

 external demibranch. This last character suggests an affinity with the Lucinidse, 

 and more particularly with the genus Montacuta placed in this family by Pelseneer ; 

 Montacuta has a single pallial suture, a very long, linguiform, byssiferous foot and a 

 shell which is in mairy respects similar to that of Jousseaumia. The ligament is 

 internal, the anterior adductor impression longer than the posterior, the cardinal 

 teeth have analogous characters, and the anterior border is longer than the posterior, 

 and as a small point of resemblance Pelseneer describes a protractor pedis ventral to 

 the anterior adductor (11, p. 203) which is paralleled by the slip of the protractor 

 in Jousseaumia. Montacuta bidentata has the habit of living in old shells, and 

 M. substriata is parasitic on an Echinid, and the former habit is suggestive of the 

 manner in which Jousseaumia may have come into association with the Sipunculid 

 inhabiting the basal chambers of corals. So similar is the shell of Montacuta to that 

 of the Erycinida? that Fischer places it in this family, but its gills not only lack the 

 external demibranch, but the filaments have considerable interlamellar extrusions, the 

 interfilamentar junctions are vascular and in these and other respects so closely 

 resemble the gills of Lucina that there can be no doubt that it should be placed, as 

 Pelseneer has placed it, in the Lucinidas. And for the same reason that Montacuta 

 is placed in the Lucinidre, Jousseaumia must be excluded from this family. Its gill 



