124 CREPIDULA. 



moved to the outside. In the first case the apex of the shell is 

 convex and brown, and the circumference white and flat; in the 

 other the apex is flat and white, and the circumference brown 

 and convex. I have seen one specimen in which the animal has 

 moved twice, and the shell has a brown tip and margin and a 

 white flat intermediate space." 



I have seen many thousands of specimens of the American C. 

 fornicata and C. unguiformis, but never noticed any showing 

 the change above recorded. 



The spinous ribbing of C. aculeata is very probably not a spe- 

 cific character, yet it occurs in connection with a general facies 

 which is easily recognizable as specific, although not readily 

 definable. 



C. FORNICATA, Linn. PL 36, figs. 1-8. 



Smooth, whitish or pink white, with radiating irregular chest- 

 nut lines often broken up into spots, interior more or less stained 

 with brown or violaceous. Length, 2 inches. 



East Coast of United States from Maine to Florida; 



West Indies, Brazil, etc. 



I figure a ribbed specimen (fig. 2). The synonymy includes 

 C. gibbosa, Defrance ; C. nautarum (Humph. ),Morch ; C. Riisei, 

 Dunker; C. maculosa, Conrad; C. depressa, Sa} r , has been 

 referred to C. unguiformis, Lam., but specimens presented* to 

 the Philada. Academy by Mrs. Say are young fornicata. C. 

 protea, d'Orb., includes this species and C. unguiformis, and is 

 said to extend from West Indies to Patagonia. I am somewhat 

 in doubt as to the West Coast distribution of this species as 

 assumed by Dr. Gray. Most of the Pacific forms which that 

 naturalist has associated with C. fornicata appear to me dis- 

 tinguishable by perhaps slight, but sufficiently recognizable 

 characteristics; C. arenata, Brod. (figs. 4, 5), however, said to 

 occur at St. Elena, West Columbia, where it was obtained by 

 Cuming, appears to be absolutely identical, and C. Patagonica, 

 d'Orb. (figs. 6, 7), differs only in being more rounded in outline 

 (in the figure) than the usual form of fornicata. G. Aplysioides, 

 Reeve (fig. 8), from Rio Janeiro, may be added. The shell 

 which Reeve has figured under the name of C. nautiloidea, -Les- 

 son, also appears to be referable here. 



