CALTPTR^IDiE. 113 



Pacific Islands, also present no marks of specific distinction. 

 It appears to be one of the ubiquitous species, of which several 

 are found in various genera, and some are known to have existed 

 far back in time. 



" The Crepidula not only undergoes the changes of form from 

 nearly flat to deepl}- arched, but from obese to elongated, which 

 every observer of the common slipper-limpet of the Atlantic 

 (G.fornicata, abundant from the icy shores of the St. Lawrence 

 to the tropical waters of the Gulf of Mexico) knows to prevail 

 in that species ; but in sculpture it may either be crowded with 

 short spines (C echinus, Brod.), or have a few radiating lines 

 of longer spines with nodulous interstices (C. hystrix, Brod.), 

 or be covered with an irregular mass of spiny knobs (normal 

 state), or lose the spines altogether in roughened strioe (smooth- 

 water form), or even become almost destitute of sculpture, like 

 some northern specimens of the stunted variety (C. Calif ornica, 

 Nuttall). Through all these changes it is recognized by its 

 spiral stomattelloid growth exemplifying a section of the genus, 

 the extreme forms of which approach Trochita ; and by its 

 beautifully waved deck-margin, which resembles a ,<-wA_^~^. The 

 pointed centre, as the shell increases in size, generally leaves a 

 characteristic line on the surface of the deck, passing up to the 

 vertex. But often the point is rounded off, and even degener- 

 ates into a broad wave. In one specimen co-ordinate with this 

 degeneracy, a sharp angle was abnormally formed on one of the 

 sides, so as to give the margin the aspect of a brace turned the 

 wrong wa}^ thus ^— --r — ' ; a very good specific distinction, if no 

 intermediate specimens had been found. A series of deck- 

 margins belonging to this and the following species, will be 

 found represented on plate 8 of the British Association Report, 

 Fig. 1, f 3, g. The best means of distinguishing the species of 

 slipper-limpets from each other was found to be the shape of the 

 nuclear portion and the mode of growth of the very young shell. 

 Wliatever be the abnormal character of the adult, it did not 

 appear that the offspring had a tendency to the same degenei'acy, 

 but rather to the resumption of the normal type. In the case of 

 local varieties, the peculiarities are reproduced, because they 

 depend on circumstances which affect all alike. But in such cases 

 as those under consideration, where the extremes and all the 



