42 University of California Publicatians in Geology [Vol. 12 



that is probably more highly specialized than any other Pacific Coast 

 species known. During its generic history (see fig. 4) several sub- 

 sidiary groups have branched off from the main Astrodapsis tumidus 

 group. In the order of appearance, these subdivisions of the genus 

 are the groups characterized by A. antiselU Conrad, A. whitneyi 

 Remond, A. amoldi (Pack), and A. fernandoensis Pack. Each forms 

 a distinct group possessing common characters, their development, for 

 the most part, taking place in certain separate geographic provinces. 

 These groups are not separate subgenera, for the reason that each is 

 bound closely to the main genus Astrodapsis by more important char- 

 acteristics than those which distinguish the groups themselves. The 

 major characters which tie these groups together are: (1) the elevated 

 petaliferous portions of the ambulacra separated by grooved inter- 

 ambulacra ; (2) the central or subcentral apical system, possessing 

 four genital pores; (3) simple ambulacral furrows; (4) an infra- 

 marginal or marginal, but never supramarginal, periproct ; (5) the 

 extension of the petals to, or very near, the margin; and (6) the 

 broadening of the ambulacral plates a considerable distance inward 

 from the end of the petal. The group characters will be given in their 

 separate discussions. 



The development of astrodapsid characters is rapid, for within 

 the same strata where the true Astrodapsis brewerianus (Remond) 

 occurs, a variety of this species is found possessing slightly raised 

 petals, which is one of the characters of the true Astrodapsis. The 

 next major astrodapsid feature acquired, is the depressed inter- 

 ambulacral areas. An incipient stage of this character is noticeable 

 in A. breiverianus. This species also shows more or less elevated 

 petals in every specimen. Up to this time the development has been 

 along one main line, but subsequent to this period the evolution of 

 this genus becomes more varied, and offshoots from the central stock 

 commence. The first of these is A. antiscUi Conrad which, though 

 closely resembling A. cierboensis Kew, is in a stage of development 

 that has taken place at a later time in a different region, the Salinas 

 Valley area. A. antiselU subsequently gave rise to the two forms 

 A. margaritanus Kew and A. ornatus Kew, the three species being the 

 first of the group to form one of the main branches in the evolutionarv 

 tree of the genus Astrodapsis. The latter two species correspond in 

 degree of evolution to A. tumidus Remond of the San Pablo Bay- 

 Mount Diablo province. A margaritanus, A. ornatus, and A. tumidus 



