pilsbry: on the genus ckyptoplax. 155 



equally young C. striatus we find a similar sculpture, except that the 

 riblets are more beaded. When the tegmentum has attained the size 

 just indicated, an abrupt change supervenes ; the lateral riblets 

 become more or less undulating, and converge forwards, meeting at 

 the ridge, where the defined dorsal tract of the preceding stage has 

 now disappeared. This sculpture lasts until a length of about 45 mm. 

 is attained by the tegmentum of valve viii (which I am simply using 

 as a standard, the stages varying somewhat with the valve, and 

 slowest, or lagging in front). Then another abrupt change appears, 

 the dorsal tract being replaced on the median and posterior valves by 

 a median sulcus, and the lateral riblets by irregular wrinkles parallel 

 with the lateral margins of the tegmentum. These stages are shown 

 in Figs. 12, 13, and 14 of PI. XIV. Fig. 12 shows the first and second 

 stages, up to the beginning of the third, the total length of the 

 tegmentum in the valve figured being 4-5 mm. Fig. 13 represents 

 valve vii of an older specimen, in which the anterior half of the 

 tegmentum has assumed the sculpture of the adult. Fig. 14 shows 

 the adult valve, in which the early stages in the development of the 

 sculpture have been removed by erosion. Length of tegmentum 

 8*5 mm. 



Or to tabulate the stages, using Hyatt's nomenclature : — 



Stage. Valve-sculpture. 



Infancy, or nepionic. Dorsal area distinct ; latero-pleural tracts with parallel riblets. 

 Youth, or neanic. Dorsal area disappearing ; latero-pleural tracts with waved 



or irregular riblets. 

 Maturity, or ephebic. A dorsal sulcus ; latero-pleural tracts with converging rugae 



parallel to the lateral margins. 



Now the stage of maturity in C. larvccformis corresponds with the 

 gerontic or senile stage in C. striatus ; old specimens of that species 

 (PI. XV, Figs. 20, 23) showing one or two lateral rugae replacing the 

 longitudinally radial, irregular wrinkles of the merely mature valve. 

 In other words, what is a senile characteristic in C. striatus has 

 become a characteristic of the next earlier stage in C. larvatformis 1 



The irregular rugae of mature C. striatus answer to the irregularly 

 waved wrinkles of the neanic C. larvceformis. If the latter species 

 ever had a distinctly granose-ribbed stage, like the earlier stage of 

 C. striatus, it will be shown by younger specimens than mine, or it 

 has been lost. 



It appears probable, from the study of young Crt/ptophx, that the 

 sculpture in that genus was primarily that of the less modifii d 

 Acanthochitidae, such as Acanthochites (JVotopku) Mattheivsi, in which 

 the sculpture consists of longitudinal riblets, granose near the beaks. 



Keturning to C. larvceformis, it may be noted that the valves 

 decrease gradually in width from the first to the fifth, which is 

 narrowest ; then there is a slight increase to the seventh, the 



1 In Chitons generally the senile stage is characterized by crowded rugae parallel to 

 the lateral margins of tbe valves, and partial or complete eifacement of other 

 sculpture there, erosion ol the valves being an incidental feature attending age. 



