THE NAUTILUS. 39 



usually are. It measures 45 mm. in length and is of a dark 

 purplish color, with a dull yellowish margin; the structure of 

 the shell is vesiculate, making it thin and brittle; umbones 

 smooth followed by a radial sculpture, which soon changes to 

 irregular, concentric laminae. At this point the growth of the 

 shell was arrested and commenced to grow at right angles to the 

 hinge; byssal opening large, affecting both valves. The pallial 

 line is conspicuously raised, forming a deep, nacre-lined body 

 cavity; from about the middle of the pallial line and extending 

 to the margin of the shell is a median, longitudinal ridge. The 

 object of this ridge seems to be that of strengthening the thin 

 vesiculate portion of the shell, for it is much more prominent 

 in the smaller than in the larger and thicker species, including 

 the two "hammer oysters." This ridge is not present in 

 Pteria. 



From the descriptions and figures given by Reeve, this species 

 cannot be satisfactorily separated from several species from the 

 Pacific, especially Malleus vesiculatus from Isle of Plata, West 

 Columbia. It also resembles except in color M, rufipmictalus and 

 M. aquatilis from the same locality. 



The metropolis of the Malleaceae being the Central Pacific, 

 their presence in the Antillean waters might possibly be due to 

 water connection by the Isthmus of Panama during the late 

 Eocene or early Oligocene, a period when so many of the ana- 

 logous species now living on the west coast of Central America 

 and in the West Indies, probably had a common origin, but its 

 occurrence in the Mediterranean makes this theory less plausi- 

 able. 



Explanation of Plate III. 



Fios. 1 and 2. Fandellacandeana {d^ Orb.). Bermuda. Speci- 

 men in the Museum of Comparative Zoology. 



Figs. 3 and 4. FundelUi candenna (d'Orb. ). From a photo- 

 graph of d'Orbigny's figures. 



Fui. 5. Malleus rufipuyidatus Reeve. From a photo- 



graph of Reeve's figure. 



F'lci. 0. Malleus vesiculatus Reeve. From a photograph 



of Reeve's figure. 



