NO. 7 PTYCHOPARIID TRILOBITES — RASETTI 7 



gap between Piazella pia and species referred by Resser to Ptycho- 

 parella, Antagmus, and Onchocephahis. 



Sombrerella Lochman, 1948 {Somhrerella mexicana Lochman). 

 No species other than the type seem to possess the same combination 

 of characters. 



In addition, Lower Cambrian ptychopariids have been referred to 

 Ptychoparella Poulsen, 1927, by Resser and Lochman. The writer 

 disagrees with this use of the genus. The type species is Ptychoparella 

 brevicauda Poulsen, collected from erratic boulders tentatively indi- 

 cated by Poulsen as "Ozarkian," now known from the faunal asso- 

 ciations to belong in the Middle Cambrian (presumably Bathyuriscus- 

 Elrathina zone). Ptychoparella is one of the generalized ptychopariids 

 abundant in beds of that age, and as far as known from the scarce 

 material available, there is little to differentiate it generically from 

 such western forms as Ehmaniella, Clap pas pis, Pachyaspis, etc. Pos- 

 sibly, when Ptychoparella becomes better known, one or more of these 

 genera will fall in its synonymy. At present it seems inadvisable to 

 abandon generic names founded on species of well-known morphology 

 and stratigraphic position in favor of a genus based on a species of 

 uncertain age. Hence the use of Ptychoparella should be confined to 

 the type species, and it seems least of all justifiable to refer Lower 

 Cambrian species to it. 



Another genus of questionable age is Perinietopus Resser, 1937. 

 The type species, Conocephalites arenosits Billings, was collected from 

 beds of doubtful stratigraphic position. The type cranidium appears 

 similar to Elrathia, Ehmania, and other Middle Cambrian genera. 

 Use of this generic name should be restricted to the type species. 



Some of the species here discussed supply information on the 

 changes in cranidial features that occur during growth. The most 

 general and conspicuous of these changes are the decrease in the 

 length of the glabella relative to the entire cranidium, and the in- 

 crease in the rate of tapering of the glabella. Such changes may still 

 affect cranidia which presumably correspond to holaspid stages (see 

 illustrations of Periomma gaspensis and Periommella yorkensis). 

 Hence great care must be exercised in comparing the proportions of 

 individuals of different sizes. Doubtless specific differences have been 

 based on such growth features. 



The writer must correct Lochman's (1947) statement that Lower 

 Cambrian trilobites in general have a thinner test than later forms. 

 While this is certainly true for the olenellids, it does not apply to the 

 corynexochids and ptychopariids, which may have as thick tests as 

 any later trilobites of comparable size. The species of Bonnia illus- 



