2 Trans. Acad. Set. of St. Louis. 



After a very careful study of the large collection of Dr. 

 Hambacb, which contains the choicest specimens to be found 

 anywhere in that particular branch, and of the Washington 

 University collection, among which is a slab with eighty-five 

 Melonites on it, I have deduced the following facts concern- 

 ing the development and growth of this family. In the 

 embryonic state a calcareous particle was deposited as the 

 nucleus of calcification of each plate of the test. As 

 the test expanded successive rings of calcareous matter were 

 deposited. While increasing laterally by the progressive 

 calcification of the individual plates, the longitudinal diam- 

 eter was lengthened by the addition of new plates between 

 the genital and ocular plates and the already existing 

 plates. That the lateral expansion took place by the depo- 

 sition of rings of calcareous substance to the edges of each 

 individual plate, is brought out very clearly in weathered 

 specimens, well defined lines marking the limits of the differ- 

 ent areas. Such specimens are figured on Pis. V. and VI., 

 figs. 9 and 10, where the older deposit is shaded, while the 

 more recent is left unshaded. 



Turning our attention to the development and structure of 

 the interambulacral areas, we find Jackson and Jaggar in their 

 paper on " Studies of Melonites multipora," * making the 

 following statement : * ' The interambulacra of the adult, when 

 perfect, consist of two plates at the ventral termination, as 

 shown in three areas, A, C and I, of plate 2, figure 2." In 

 my researches, I find that ventrally the interambulacra always 

 terminate in three plates, when the specimen is perfect. In 

 all the specimens with the teeth in place, this is invariably the 

 case as may be. readily seen from figs. 6, 8, 12 and 13. This 

 feature is shown most admirably by fig. 8, pi. IV., from which 

 specimen the drawing of the teeth (fig. 8 f, pi. IV.) was made. 

 The specimen shows plainly how the sides of the terminating 

 plates come close up to the jaws, making it impossible for any 

 plates to have dropped out. Besides the specimens I have 

 mentioned, I examined a large amount of material, only to 

 find that the aforesaid is always the case when the specimens 



* Bull. Geol. Soc. Amer., vol. 7, p. 142. 



