4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



Bolaspidella housensis (Walcott) 



Ptychoparia housensis Walcott, U. S. Geol. Surv. Bull. 30, p. 201, pi. 25, 

 fig. 5, 1886. 



Middle Cambrian, Wheeler; (loc. 4) Antelope Spring, House 

 Range, Utah. 



Holotype. — U.S.N.M. no. 15443. 



BONNIELLA, n. gen. 



Members of the long-ranging trilobite group to which Bonnia, 

 Kootenia, Olenoides, and other genera belong are numerous in Lower 

 and Middle Cambrian rocks. For example, Bonnia, wherever present, 

 is usually represented by many species and thousands of individuals. 

 A monographic study of this trilobite group has now gone far enough 

 to warrant a few generalizations. In the first place few of the genera 

 can be determined by cranidia alone, so that genera and even species 

 must be made on pygidia, and the cranidia matched as nearly as pos- 

 sible by association, surface markings, or by proportionate dimensions. 

 In the second place the genera grade one into the other, so that 

 arbitrary lines of separation must be chosen, which for instance be- 

 tween Bonnia and Kootenia is the degree of fusion of pygidial pleural 

 segments, and between Kootenia and Olenoides — both with pleural 

 separations visible — is the unequal width of the two portions of the 

 pleuron resulting from the diagonal course of the pleural furrow. A 

 third generalization apparently warranted is that marginal or axial 

 spines have nothing more than a specific value. Within a genus the 

 marginal spines may vary from mere waviness of the margin near the 

 anterior angles to a full suite of spines, sometimes longer than the 

 pygidium. These and other modifications of the fundamental group 

 characteristics tend to come and go with the passage of time. 



Another modification not mentioned in the foregoing lines is the 

 development of a flattened marginal extension instead of spines, which 

 rather strangely seems to be accompanied by expansion of the anterior 

 portion of the glabella. This tendency in the Lower Cambrian has 

 given rise to the forms grouped under the new generic name Bomiiella, 

 and in late Middle Cambrian it has produced Holteria. 



Bonniclla is primarily characterized by the pygidial flange back of 

 the second segment. Exclusive of this feature, the pygidium is typical 

 of Bonnia in general appearance and in the presence of marginal 

 spines at the anterior angles, but it has less complete pleural fusion 

 than in the majority of species belonging to Bonnia. The cranidium, 

 as in Bonnia and other members of the family, is quadrate. In 



