Nov., 1918] Structure oj A geJacrinites and Streptaster 75 



One is often under the impression that a fossil, being ancient, 

 must have lived a long time. This, of course, is not at all 

 necessary. Among the numerous specimens with their aboral 

 faces up collected at Lawshe, Figure 10, Plate IV, shows some 

 30 which had been seated upon a valve of Ischyrodonta ovalis(?). 

 The largest of these is not 3 mm. in diameter and the smallest 

 less than half that. On the same rock, Plate IV, Figure 14, is 

 another group of five individuals which cover a similar shell 

 nearly as fully. These range from 10 to 12 mm. in diameter. 

 On another rock fragment I have one Agelacrinite approaching 

 the larger size and several small ones about it, none over 3 rnm. 



There is no doubt in my mind that the specimens of 3 mm. 

 or less attained that diameter during their first season's growth 

 and that the ones which survived for a second season reached 

 the larger dimension. A specimen which measures 17 mm. 

 from this same locality may have grown for three seasons. 



Dr. A. D. Mead, in the American Naturalist for January, 

 1900,' showed that in the case of the starfish the rate of growth 

 depends definitely on the food supply. He there gives a diagram 

 of two starfishes of the same age (two months) the one about 

 the size of a pinhead and the other 2.5 inches in diameter. 

 The size also determined the sexual advancement of these 

 starfishes and those of a certain size were mature, no matter 

 what the length of time needed to reach that size. "We are 

 warranted in inferring therefore that well nourished starfish 

 arrive at sexual maturity and breed before they are a year old. " 



If we suppose these Agelacrinites were able to reproduce 

 the second season we have evidence of a colony located at the 

 top of the Richmond sea-deposit near Lawshe which must have 

 had favorable conditions for at least two seasons of growth and 

 probably three, since there are three well defined sizes repre- 

 sented on the clam valves. Of these the smallest specimens 

 (up to 3 mm.) are by far the most abundant, hundreds of them 

 being found on a single piece of rock. The size 10 to 12 mm., is 

 rather common and the very largest, 17 mm., are represented 

 by few individuals. 



The smallest specimen identified was two-thirds of a milli- 

 meter in diameter and showed the aboral face. The smallest 

 specimen oral side up is 1.5 mm. in diameter. 



