DESCRIPTION OF AN ARTICULATE OF DOUBTFUL RELATIONSHIP FROM 

 THE TERTIARY BEDS OF FLORISSANT, COLORADO. 



JtEAD AT WASffimiTON, At'lilL 20, 188;! 



By Sami'ei. H. Scuddek. 



Amoug the remains of auimals in my bauds I'ouud iu the aucient lake basin of Florissant are 

 about forty specimens of an onisciform arthropod, about a centimeter in length, whose affinities 

 have jiroved very perplexing. This does not result from poorness of preservation, for among the 

 numerous specimens apparently all the prominent external features are found completely pre- 

 served, and even the course of some of the internal organs may occasionally be traced; but it 

 presents such anomalies of structure that we are at a loss where to look for its nearest kin. 



It appears to be an aquatic animal. Its body consists of three large subequal thoracic joints, 

 and an abdomen about half as large again as any one of them, with occasional indications of a 

 feeble division into four segments. These are the oidj' jointed divisions that can be found in the 

 body, there being no distinct head. The thoracic segments are so considered because each bears 

 a pair of legs, which occur nowhere else. Their dorsal plates are large, flat longitudinally, and 

 arched transversely; smooth, and deeply and narrowly notched in the middle of the front margin. 



Fig. 3. 



Fig. 2. 



Fig. 1, dorsal view; tig, 2, lateral view: fig. 3, transverse sectional view of Planoceph- 

 alus aneUoidea from the oligocene of Florissant, Colorado, restored, and magnified about six diam- 

 eters. 



Fig. 1. 



The tirst plate, in which the median notch is more conspicuous and open than in the others, also 

 narrows and becomes more arched in front, so as to form a sort of hood. The legs are very broad 

 and compressed, and adapted to swimming, which was apparently their use, as there would be no 

 need of such compression to crawl into chinks when the body is so much arched. They consist of 

 a femur, tibia, and two tarsal joints, terminated by a single curved claw. The femur is very 

 large, subovate, inserted (presumably by a coxa) in large cavities, those of opposite sides sepa- 

 rated by their own width, and situated a little behind the middle of each segment. The tibia is 

 also very large and subovate, but more elongated and squarer at the ends, being about twice as 

 long as broad, and fringed on the anterior edge by a row of delicate hairs as long as the width of 

 the joint. Of the two tarsal joints, the basal is a little the larger, being both longer and stouter. 

 Each is armed at the tip internally with a tolerably stout spine of moderate length, and together 

 they are a little longer than the tibia, much slenderer, and quadrate iu form. The terminal claw 



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