222 BULLETIN OF THE 



structure barely visible on extreme of tbe left valve only; interior of adult 

 marked by a flat thickened margination, especially in the left valve, extending 

 parallel with the basal margin ; auricular crura elevated into u low ridge or 

 blunt tooth on each side ; hinge margin furnished with a lanceolate area on 

 each side of the cartilage pit which is finely deeply closely grooved in a direc- 

 tion vertical to the hinge-line, the projections between the grooves interlock- 

 ing so strongly as to cause the hinge-line of one valve sometimes to break off 

 bodily, rather than separate from the opposite valve, when one is trying to 

 open a pair, even when the soft parts are absent. The shell is prettily varie- 

 gated with mottlings of red, brown, and yellow on a creamy ground, both 

 valves participating, but the upper one rather the brighter in most cases. 

 Alt. 8.5 ; Ion. 9.0 ; max. diam. 2.0 mm. 



This shell has been dredged by Professor Verrill in 80 to 317 fnis. off 

 Martha's Vineyard. Specimens were obtained by Sigsbee off Havana in 450 

 fnis., and at Station 36 in 84 fms. 



The figure on Plate VI. fig. 5. representing the interior of the young fry 

 of Janira hemicyclica, equally Avell represents the very young of this species, 

 which at this stage can hardly be distinguished from the other except by the 

 external sculpture, which is often absent, or by the rug* of the hinge. 



Because the internal lira; occasionally fail, or are late in being deposited in 

 certain individuals of Propeamufshim, Dr. Jeffreys formed the opinion that they 

 were of no importance as a specific character. In this manner he was led into 

 several errors, from judging merely by the exterior of some of these small shells. 

 In the present case the external sculpture is much like that of Amusium fenes- 

 tratum Forbes, but finer and more regular. Both have similar varieties of 

 color. Dr. Jeffreys was thus led to unite them, although they belong to differ- 

 ent sections of the genus, as also in the case of A. Hoskynsii and Pseudamnsium 

 pustulosnmYerviW, an error entailing some serious results in the matter of areal 

 distribution. Professor Verrill followed Dr. Jeffreys until a special study of 

 the gi-oup enabled me to call his attention to the discrepancies existing between 

 the two species. 



The peculiar arrangement of grooved and comigated areas on the hinge-line 

 is of interest in connection with the development of hinge characters. It is 

 very common in the fry of various Pectens. It is well developed in the very 

 young P. magellanicus. I have found traces of it in many of the minute 

 specimens oi Pecten I have examined, though often very faint, and in very few 

 does it reach the development attained in the present species. It was the adult 

 and peraianent characteristic of the extinct genus Neithea, and traces of it are 

 even recognizable in some .species of Janira or Pecten in their adult condition. 

 When I first discovered it in the present species, I naturally supposed I had 

 come upon the type of a new generic division, but I have since found it in the 

 young of Pectens of nearly all sections of the genus in its widest sense. On 

 the other hand, in some species even the very young have hardly a trace 

 of it. 



