80 IOWA ACADEMY OF SCIENCES. 



The species in question which is of the general type of 

 Encrinurus imnctatus Wahlenburg, is well represented in the 

 collections by two perfect, or nearly perfect, specimens and by 

 scores of cephelons, moveable cheeks and pygidia, occurring 

 both as external moulds and internal casts. 



The nearness of the fossiliferous stratum to the top of the 

 quarry brought it well within the zone of weathering. The 

 laminas of the rock were parted and the fossils thus disengaged 

 with a single stroke of the hammer, and without any picking 

 and cleaning that might mingle artificial with the delicate 

 natural markings. The latter are exceptionally well preserved. 

 To speak of the pygidia only, the caudal spine is shown in 

 several specimens, the ninth pair of pleural are usually 

 distinct, and even a tenth pair may sometimes be seen as min- 

 ute ridges nearly aligned with the axial lobe and ending upon 

 it in a tubercle. Oi: the segmental lines on the mid-lobe as 

 many as thirty-one have been counted with the aid of a magni- 

 fyiog glass, and in seven specimens over .twenty-five are thus 

 vi&ible, and in several specimens eight and even nine axial nodes 

 have been observed. 



The investigation has thus been specially favored in the 

 number and perfection of the specimens at hand. The promi- 

 nence also of the large rounded anterior tubercle affords a sure 

 ground which would be lacking if the investigation were 

 carried over to the less distinct tubercles on the broad pleural 

 annulations. In the same way the size of the specimens is of 

 advantage. The largest twenty- three mm. in length and 

 width, slightly exceeds in these dimensions the largest Encrin- 

 urus the author has seen figured or described. From this size 

 the specimens rarge to a minimum of eight mm. in length and 

 breadth. In several of the smaller pygidia, the axial lobe is 

 slightly more convex and the central longitudinal space 

 between the discontinuous segments is more or less obscure. 

 The first nine segments in especial, are plainly continuous. 

 While it is not thought that these are specifically distinct, they 

 are separated in the following table by being marked with 

 a star. Excluding these and considering the remainder whose 

 specific identity can not be questioned, the following variation 

 is observed: 



