ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 293 



keeled appearance to the lower portion of the whorl in some specimens; aperture subquadrate; outer 

 lip thin; c(jlunielhi truncate, s|)irally sculptured without. 



Dimensions. — Long. lonini.; hit. 2.5 mm.; dell. 18 degrees. 



This species is intermediate between B. fit<Kini\ ami />. nsperum, having the 

 .sculpture of the former on tlie body-wliorl and the sculpture of the latter on the 

 apical whorls, while the intermediate whorls have a fjradation sculpture. Speci- 

 mens identified hy Dr. Dall. 



Not uncommon in the lower San Pedro series at Deadman Island and San 

 Pedro; rarr in tiic upper San Pedro series at Deadman Island, San Pedro, Los 

 (^erritos, and Crawlish George's. Found in the Pleistocene at the halh-house, Santa 



I'arhara. 



The specimen figured is from tiie lower San Pedro series at Deadman Island, 

 and is now in the collection of Delos Arnold. 



Living. — Monterey to San Diego (Cooper). 



Pleistocene. — Santa Barbara (Cooper): San Pedro; Santa Barbara (Arnold). 



One of the laws of evolution is that of acceleration of development, which has 

 been formulated by Alpheus Hyatt,' as follows: — 



"All modifications and variations in progressive series tend to a})pear first in 

 the adolescent or adult stages of growth, and then to be inherited at earlier and 

 earlier stages, according to the law of acceleration, until they either become embry- 

 onic or are crowded out of the organization, and replaced in the development by 

 characters of later origin." 



This means that theoretically each organism in its ontogeny, or life hi.story, 

 ought to go through stages of growth corresponding to all of its ancestors, and that 

 these stages ought to appear in the order of its ancestral forms. This law is exempli- 

 fied by tlie genetic series, consisting of B. nsperum, B. quadrijilatum and B.Jilosum. 



First, we have B. asperum, which is the only species of this genus found in 

 the Pliocene at Deadman Island. It has a characteristic nodose-cancellate sculpture, 

 caused by two series of ridges, s[)iral and transverse, and this sculpture extends back 

 from the body-whorl through the intermediate to the apical whorls. Thus, in this 

 species the same characteristic sculpture is found in the larval stage (which is rep- 

 resented by the apical whorls), the adolescent stage (intermediate whorls), and in the 

 adult stage, which is represented by the penultimate and body-whorls. 



Next we have B. quadvifilntum (Plate IX, fig. 2), which is first found in 

 the lower San Pedro beds of the Pleistocene, and which, it is reasonable to suppose, 

 developed out of Bitthnn usj^eriim. At any rate, this species is not found in the 

 Pliocene, but is quite abundant in the lower San Pedro. The sculpture of this 

 species is as follows: Apical whorls nodose-cancellate; intermediate whorls with the 

 transverse sculpture becoming less prominent as the whorls become larger; penulti- 

 mate and body-whorl generally with four prominent spiral threads or ridges, but with 



> A. Hyatt, "Qenegls of the Ailetldn," Preface, p. Ix. 



