48 



CHESTER OSTRACODES OF ILLINOIS 



those from the Chester formations, has 

 shown the need for finer discrimination 

 among the forms hitherto grouped under 

 this genus. 



The holotype A. rugosus Girty is char- 

 acteristic of a group of species occurring in 

 the Chester series and the Pennsylvanian 

 and Permian systems. These species possess 

 a single node or swelling, centrally located, 

 and two or more carinae or false keels, par- 

 allel or subparallel to the free margins of 

 the carapace. A. centronotus Ulrich and 

 Bassler is another good representative. 



Another group of species is intermediate 

 between the genotypes of Arnphissites and 

 Kirkbya. This group possesses one or more 

 carinae and a prominent and characteristic 

 kirkbyan pit but lacks the wing-like termin- 

 ations of Kirkbydj as well as the central 

 node, and the prominent kirkbyan shoulder. 

 The surface reticulations are often coarser 

 than those found on most species of Ajn- 

 phissites. 



The third group which is especially char- 

 acteristic of the Chester series and of many 

 Pennsylvanian formations, possesses three 

 nodes. Some species develop swellings in the 

 antero- and postero-ventral corners, thus 

 approaching a five-node form. The terminal 

 nodes near the dorsal margin may be short 

 and similar to the central node or, as is often 

 the case, they may be elongated to form 

 ridges which are subparallel to the anterior 

 and posterior free margins. 



All three groups possess the typical kirk- 

 byan characters, namely : the subcentral pit, 

 straight hinge line, one or more carina or 

 false keels, and a pronounced reticulated 

 surface. It is proposed to restrict the name 

 Amphissites to the forms characterized by 

 the genotype A. rugosus and recognize the 

 other two groups as genera for which the 

 names Ectodemites and Polytylites are pro- 

 posed. The subfamily Amphissitinae would 

 include, in addition to these groups, the 

 allied genera Knightina, Balantoides, and 

 Ulrichia at least in part. There is some 

 question concerning the actual occurrence 

 of a sulcus between the two nodes of Ul- 

 richia, and if one is not present, this genus 

 should, ^ beyond question, belong to the 

 Kirkbyidae and probably to the subfamily 

 Amphissitinae. 



The several criteria for orientation of 

 ostracodes have been variously interpreted 

 for the Amphissitinae by different authors, 

 so that considerable confusion is prevalent. 

 In the original description of the genotype, 

 Amphissites rugosus J Girty (1910) regarded 

 the overlap as right over left. In this he was 

 followed by Roundy (1926) who figured 

 the genotype for the first time. Roth ( 1929) 

 reversed this orientation, emphasizing the 

 position of the teeth on the cardinal extrem- 

 ities of what he considered the left valve. 

 Roth was followed by Knight (1928), Kel- 

 lett (1934), and others, but Bradfield 

 (1935) and Payne (1937) reverted to the 

 original orientation of Girty. Some authors 

 have confused the issue by following one or 

 the other of these methods and then incon- 

 sistently orienting their figures. The original 

 orientation of Girty has been followed be- 

 cause this places the dorsal shoulder poster- 

 ior, making the greatest height and width 

 of the shell posterior. The swing of the 

 valves, which is very slight if present in this 

 group at all, is believed to be of little value 

 in orienting these forms. 



Genus Amphissites Girty, 

 restricted Cooper 



Amphissites, in part, of authors. 



Kirkbyidae with single, usually large 

 node, located centrally or nearly so, be- 

 tween anterior and posterior ends, com- 

 monly slightly above line midway between 

 dorsal and ventral margins; small elliptical 

 pit located at postero-ventral edge of node ; 

 valves subequal, right slightly overlapping 

 left ; one or two false keels paralleling ven- 

 tral margin but joining at cardinal extrem- 

 ities; another carina, not completely encir- 

 cling ventral node, joins a straight ridge 

 parallel to hinge line ; short ridge joins inner 

 carina at about 90°, giving cardinal area ap- 

 pearance of narrow shield, with broadest 

 end posterior (see pi. 9, figs. 1, 19, 34) ; 

 surface reticulate. 



Amphissites carinatus Cooper, n. sp. 



Plate 9, figures 19-21 



Amphissites centronotus? Croneis and Gale, 1938, 

 Denison Univ., Jour. Sci. Lab., Bull. 33, 

 p. 270, pi. 5, fig. 9, Golconda formation. 



Carapace short, thick and high, coarsely 

 reticulate, marked by prominent carinae; 

 node large, oval, slightly elongate horizon- 

 tally, and lying just above center of shell 



