136 STUDIES IN EVOLUTION 



ventral, continuous ; suture marginal or ventral. Eyes wanting. 

 Thorax composed of from two to four segments, with grooved 

 pleura. Cambrian and Ordovician. 



Including the genera Agnostus Brongniart and Microdiscus 

 Emmons. 



The genera in this family are primitive in their form and 

 structure, as shown by their ventral free-cheeks, marginal 

 or ventral suture, elongate cephalon, and large pygidium. 

 Some species have spines at the genal angles, corresponding 

 to the interocular spines of Holmia and young UUiptocephala, 

 and not to the spiniform projections of the free-cheeks. 

 From their abbreviated thorax and progressive loss of annula- 

 tions on the glabella and axis of the pygidium they must also 

 be considered as degraded. Microdiscus, the earlier genus, 

 has three or four free segments, and in some species {M. spe- 

 ciosus Ford) preserves the normal pentamerous glabella and 

 annulated pygidial axis, while the later genus, Agnostus, has 

 but two free segments, and has lost the annulations of both 

 glabella and pygidium. Matthew^'' has described the pro- 

 taspis stage of Microdiscus, which agrees with the similar 

 stage of Ptyclio'paria and Sao. 



Fully a dozen generic names have been proposed for forms 

 of the general type of Agnostus, but none of them has ever 

 come into current use. Nine were first published by Corda,^^ 

 but as Barrande^ subsequently showed that one was based 

 on an Orhicula, another on a poor specimen of ^glina, and 

 three others on a single species, this grouping soon fell into 

 disuse. Moreover, Barrande was inclined to give no generic 

 value to the form and lobation of the glabella, and therefore 

 all the species were placed by him in the single genus 

 Agnostus. At the present time more weight is given to the 

 characters of the glabella and pygidium, as indicating generic 

 differences in dorsal and ventral structure, so that further 

 study may show the desirability of restoring such of 

 Corda's names as were founded upon natural groups of this 

 family. 



