342 



LOWER INVERTEBRATES. 



Of the Strepomatinse, about five hundred nominal species have been described, 

 but until we know more of the anatomy, the life history, and the variations of these 

 forms, nothing definite can be known of their classification. Here is a wide field for 

 study open to those students of the central portions of the United States, which will 



Fig. 434. — Anculotus 

 prcerosa. 



Fig. 435.— .4mcu- 

 lotus plicaia. 



Fia. iSe. — Lithr 

 asiaj 



Fig. 437. —Gonio- 

 basis depygis. 



Fig. 438. — GoMio- 

 basis impressa. 



be productive of great results. Of the development we know absolutely nothing, not 

 even if a veliger is formed. Ten genera and sub-genera are recognized, and the great 

 bulk of our species come from the Ohio River and its tributaries. Species of Ancu- 

 lotus are found in the Potomac and Susquehanna, while others of the family have been 

 introduced into the Erie Canal. None are known to occur naturally in New England, 



though a few years ago some were introduced into Lanes- 

 boro (Mass.) pond, where they appear to thrive. 



Of the species we have but little to say, and will simply 

 let our figures, which represent the prominent genera, speak 

 for themsehes. The first species of lo described was re- 

 garded by the early American naturalist, Thomas Say, as a 

 fresh-water species of the genus Fusus, with which it has in 

 reality nothing to do. All of the species of this genus occur 

 in a very limited tract, and have so far been found only in 

 the mountainous regions of western Virginia and eastern 

 Tennessee. Most of the other genera have a rather re- 

 stricted distribution, Goniobasis being the most widely dis- 

 tributed; it contains over half of the known species. Schi- 

 zostoma is very similar, but may be separated by a notch on 

 the posterior edge of the outer lip, produced in some way 

 as yet not understood. This genus occurs only in northern 

 Alabama, in the streams which flow into the Tennessee River. 



The Pteamidblltd^ is a small family of marine molluscs, with a 

 long, slender shell, in which the columella has frequently one or more 

 prominent folds ; the eyes are sessile, the proboscis retractile, and the 

 tentacles either broad or long and slender. In many the lingual teeth 

 have entirely disappeared, owing to their parasitic habits. 



Pyramidella is a tropical genus of littoral molluscs, the members of -^^^ m.-scu- 

 which burrow along just beneath the surface of the mud. The members 1°^*™" »»'*- 

 of the allied fossil genus Nerinma were much larger than any of the ex- 

 isting members of the family. The species of Odostomia are numerous and widely dis- 

 tributed, the genus being represented by several species on our coasts. Some of these 

 usually live beneath stones, moving in a ^•ery slow manner, while others are usually found 

 on the shells of scallops {Pecten) and but rarely anywhere else. Whether they live a 

 semi-parasitic life or are commensals has not been settled. Stylifer is much more of a 



Fig. 439. — lo spinosa. 



