240 GENERAL GEOLOGY. 



are not unfrequently found; and species of the Heteropod 

 division of the order appear again in these strata, none 

 having yet been found in any of the Iowa strata that 

 intervene between them and the Kinderhook beds. Lamelli- 

 hrancMates are found in some places in considerable num- 

 bers, but BraxMopods are the most characteristic fossils of 

 the formation, Productus, Clionetes, Spirifer, and Athyris 

 being the most characteristic genera. Polyzoa are also 

 found, but are not abundant. 



Radiates are very rare in the I^ower coal-measure strata, 

 the genus Zaplirentis only, being thus far recognized. 



Protozoa. JSTo specimens of these lowly organisms have 

 been detected in the Iowa strata of this formation, but as 

 they are not uncommon in the Middle, and abundant in the 

 Upper coal-measures, it is probable that they exist in the 

 Lower also, because many of the species of fossils pass 

 uninterruptedly through the whole series of strata which 

 compose the coal-measure group. 



Fossil Plants. With the possible exception of some 

 specimens of silicihed wood that have been found in the 

 Lower coal-measure sandstones, which seems to have 

 exogenous structure, all the remains of terrestial vegetation 

 yet discovered in this formation probably belong to the class 

 Acrogens. The most conspicuous of these remains, and the 

 most characteristic of the Lower coal-measures, is one or 

 more species of Lepidodendron and another of Catamites. 

 Imperfect specimens belonging to these two genera are 

 occasionally found in all parts of the region occupied by 

 these strata, but the best collections yet obtained were made 

 by Prof. T. S. Parvin, from the sandstones of the large coal- 

 measure outlier at Muscatine, and during the present season 

 he has obtained from the same place some interesting bodies 

 that are probable the spore-cones of Lepidodendron. Speci- 

 mens of Calamites, as well as of several species of ferns, are 

 found among the strata of all three of the coal-measure 

 formations, but the genus Lepidodendron seems not to have 

 existed later than the epoch of the Middle coal-measures, or 



