326 LOWER INVERTEBRATES. 



less flattened cone, usually perforated by a wide and deep umbilicus. The shells are 

 not nacreous, and a horny spiral operculum closes the usually angular aperture. The 

 animal has sessile eyes, long, retractile proboscis, and the gill chamber divided into 

 two parts. 



Solarium perspectivum has received the common names of perspective shell and 

 sun-dial shell. It is about two and a half inches in diameter, and is of a yellowish 

 hue, prettily spotted and banded with red. It comes from the Indian Ocean, and 

 specimens are found in most collections. 



The only other genus which needs to be mentioned is Phoi'us, which embraces the 

 carrier or mason-shells of the eastern seas. These forms have the habit (if habit it 

 may be called) of covering their shells with all sorts of extraneous objects, — shells, 

 stones, bits of coral, and the like. These foreign bodies are fastened by the substance 

 of the shell and doubtless are protective ; for, viewed from above, a shell thus tricked 

 out has but slight resemblance to a pi-operly conducted mollusc, and thus runs a better 

 chance of escaping the maw of the bottom-feeding fishes. Now that this peculiar 

 habit exists, we can readily see how it is retained, but the way in which it was first 

 acquired is not so readily explained. 



The mode of progression of the mason shells is rather peculiar. Most of the gas- 

 teropodous molluscs have a gliding motion, the various parts of the foot acting in a 

 manner best described by comparing it to the locomotion of a thousand-legged worm. 

 The Phori, on the other hand, have a gait like that of a measuring worm. They 

 extend the small cylindrical foot, attach the anterior portion, and then draw the hind 

 portion forward. This latter now affords a foothold; the anterior portion is again 

 extended, and the operation is repeated. This gives rise to an interrupted, almost 

 jumping movement, well adapted to the banks of broken coral and dead shells inhabited 

 by these animals. 



The ScAiAKiD^, or wentle-trap family, embraces but a single genus and about a 

 hundred and iifty species, distributed through all the seas of the world. The common 

 name is a corruption of the German word for a spiral stairway, and would be emi- 

 nently appropriate, were it not for the fact that the tread of the stairs goes the wrong 

 way. The shells are usually pure white, and composed of several rounded whorls 

 ornamented with transverse ribs, roughly corresponding to the steps of a flight of 

 stairs. The aperture of the shell is round and the edge continuous, the inner lip not 

 being formed by the columella. The active, predaceous animal has 

 a retractile proboscis, and in the existence of a rudimentary siphonal 

 fold shows an approach to the members of the next sub-order. The 

 eyes are near the outer bases of the slender, pointed tentacles. 

 Several species of Scalaria are found on our New England coasts. 

 Mr. Couthouy kept a specimen of S. gronlandica in confinement ; 

 it was rather sluggish in its movements, and fed' eagei'ly on fresh 

 beef, especially if somewhat macerated. Some of the species are 

 said to secrete a purple fluid. 



The most noted member of the family is Scalaria pretiosa, 



ma.ias.-ScaiaHapre- ^^^ precious wentle trap. This species, which comes from the 



tie-teap?"'"™ "'°°' Chinese Seas, has always been highly valued by collectors on 



account of its rarity, and a single specimen has in times past been 



sold for about two hundred dollars. Now they are much more common, and are sold 



by dealers for an average pi'ice of one or two dollars. 



