8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



In these cases there may even be two shallow furrows across the 

 glabella. The sharp and deep transglabellar furrow of Acimetopus 

 hilohatus and Calodiscus reticulatus, and the somewhat shallower one 

 of Analox bipunctata, seem to represent more advanced stages of 

 this type of glabellar lobation. A sharp transverse furrow was pre- 

 viously known in Opsidiscus bilobatus (Westergard, 1946) and Tan- 

 nudiscus tannuolaicus (Pokrovskaya, 1959). It is not clear whether 

 the transglabellar furrow in these Eodiscids, and the possibly homol- 

 ogous furrow in the Agnostids, result from modification of one of 

 the pairs of glabellar furrows, or are independent of them, represent- 

 ing a secondary segmentation. The doubt is suggested by the prob- 

 able secondary nature of the segmentation of the pygidial axis in the 

 Agnostids (Palmer, 1955). 



In Acimetopus the glabellar lobation is the most specialized of 

 any described Eodiscid, the broad transglabellar furrow possessing a 

 pair of small, rounded lobes at the sides. 



In Stigmadiscus, and to a lesser extent in Acidiscus, the two pairs 

 of lateral glabellar furrows assume the form of short, fairly deep pits, 

 a feature unusual in the family even though it occurs in other groups 

 of trilobites. 



Another unusual character for the Eodiscids is the extension of 

 the posterior portion of the glabella into an upright spine, observed 

 in Acidiscus, Acimetopus, and Bolboparia. In Acidiscus there is, in 

 addition, an occipital spine, whereas in Acimetopus and Bolboparia 

 the occipital ring is short and simple. 



Cephalic border. — A definite border furrow and border around the 

 cephalon are present in all genera excepting Analox where the lateral 

 border is missing, although an anterior border of triangular shape 

 and the posterior border are well developed. One of the striking 

 features of the border in several of the new genera is the develop- 

 ment of marginal spines at various positions. 



One pair of short, lateral marginal spines is observed in Acimeto- 

 pus, Bolboparia, Litcnnetopus, and Acidiscus birdi. Two pairs are 

 present in Acidiscus hexacanthus and Oodiscus subgranulatus. As 

 the spines vary in position, it is questionable whether they can be 

 homologized among the different species. 



The posterior cephalic border shows variable features that can 

 not be readily homologized among the different genera. In many of 

 the species it bears a pair of spines, which may vary in position from 

 less than halfway from the axial furrow to the genal angle (in Oodis- 

 cus, Bathydiscus, and Stigmadiscus stenometopus) to the proximity 

 of the genal angle as in Serrodiscus subclavatus, Acimetopus bilobct- 



