MOLLUSCS. 319 



Zimax; as shown in Fig. :J4S on a j>r(;cLMling page. In the principal genus, Arion, 

 there is a trianguhir pore at the upper posterior ]iart of the body, which readilj- sep- 

 arates it from Limax. Tlie only spe- 

 cies in the United States which un- 

 doubtedly belongs to this genus is 

 Arion fuscus, which lias been intro- 

 duced from Europe into Boston, where ' 



a colony lias existed for many years. Fui.-M\.—Anaii itucus. 



It lives in gardens, and occasionally 



strays into cellars and other dark places. It is not known elsewhei-e in America. 

 In Europe it is a common species, and its eggs are said to be phosphorescent, shining 

 in the dark for several days after being laid. In color this species is whitish or gray- 

 ish, sometimes tinged with brown. It i-eaches a length of about two inches. 



Tiiree other genera of Ai-ioninie, Ariolinia.r, ProjJii/naon, and Hc/z/jj/ii/lia, are 

 found on the Pacific coast. 



The last sub-family, the Limacina", embraces the largest pro]iortion of the slugs, 

 the typical genus, Umax., containing about one hundred species. This is the only 

 genus represented in the United States, where, besides our native species, we have 

 se\eral introduced from Euro])e. Our largest species, Umax maximus, is one of these 

 immigrants, whicii has been found in several ]ilaces in America. Its rich brown or 

 black spots and stripes upon an ashy or light brown groundwork make it a conspicuous 

 form. 



Another imported species is X. ./?««?<«, brown or brownish in color, with lighter 

 spots. This is more ((inimon than L. maximus, and is found in various Atlantic cities 



from Boston to Charleston. It lives in cel- 

 lars and in gardens, preferring the former. 

 Still more common is the smaller L. agi-es- 

 tis, which is also an introduced form. It 

 is smaller than the others, and is extremely 

 \ariable in color. It lays more eggs than 

 the two species mentioned, and the period 

 of reproduction appears to last through the 

 wai-mer months of the year. Our native Zimax cumj^estris is very common, and is 

 found in the woods and the open fields, along the sides of the roads and in gardens. 

 It is brownish gray or amber colored, and is smaller than the other species men- 

 tioned. The eggs are rather numerous and transparent, and are laid under leaves or 

 in moist earth. Dr. E. L. Mark has studied the earlier stages of the develo])ment of 

 this sjiecies; a later stage is shown in Fis;. 348. 



Another genus, Phosphorax, which is very imperfectly known, comes from the 

 Cape Yerdes. The only species is said to be phosphorescent, as is indicated by both 

 its generic and specific names {P. nocti/iicois). 



Order III. — ZYGOBRANCHIA. 



All of the gasteropods whicli follow belong to the Strei)toiieurous group, the 

 characters of which were detailed on a preceding page. In the first division, the 

 Zygobranchia, the torsion of the body has not been accom]ianied by an atrophy or 

 disappearance of the organs of the primitive left side, and we thus have the «.ills"and 



