458 M. O'Cpnnell — Ortho genetic Development 



I have taken a single species among the ammonites and 

 have studied the ontogeny in a single specimen. The 

 species is a new one and belongs to the Jurassic ammo- 

 nite fauna of Cuba collected by Mr. Barnum Brown and 

 now in the American Museum of Natural History. The 

 holotype of the species, Perisphinctes cubanensis O'Con- 

 nell, shows orthogenetic development in several shell 

 characters, but I have selected as the most striking illus- 

 tration that of the development of the costae. The entire 

 method of growth of the costae will be discussed in a 

 forthcoming paper, but the particular illustration of 

 orthogenesis is amply shown on the last two whorls of 

 the holotype. The costae are arranged in groups of three 

 which consist of one long costa extending from dorsum 

 to venter of the whorl and two short costae which extend 

 across the venter and about a third of the way dorsad 

 on the whorl. A generic characteristic of Perisphinctes" 

 is the presence, at repeated intervals on the conch, of 

 constrictions or grooves which appear as interruptions 

 to the normal costal arrangement. In Perisphinctes 

 cubanensis there are from three to five groups of costae 

 between every two constrictions or sphincters (S. in fig. 

 1). In each inter sphincterial sector of the shell the 

 costae show a progressive mode of development, each 

 group of three costae being a little in advance in two 

 characters over that just preceding. The simplest 

 arrangement of the three costae consists of a long costa 

 extending from venter to dorsum, a short one attached 

 to the long one and the third branching off from the first 

 at a point slightly lower than the point of attachment of 

 the second costa ; the second and third costae are of equal 

 strength and both are weaker than and not so thick as 

 the first. This primitive condition is shown in the 

 earlier whorls of the species and is very nearly ap- 

 proached in the first triad illustrated in fig. 1 where, 

 however, the third costa is slightly separated from the 

 first. Starting with this primitive costal grouping the 

 second set of three (fig. 1) shows the greatest strength 

 in the second costa with the first and third of equal 

 strength and both attached to the second costa and bend- 

 ing towards it, one forward the other backward, so that 

 the three appear likes the tines of a fork. The next 

 group of three shows the greatest strength still in the 

 second costa, the first being now separated off as a free 



