FREE INSTITUTE OF SCIENCE 



TERTIARY FAUNA OF FLORIDA 



polish of the protoconch is quickly lost by friction, to which sessile mollusks 

 in shallow water are particularly liable, and one needs, to secure a satisfactory 

 study of this early stage, very young and lately attached individuals. In other 

 species, such as C. lactuca (from one hundred fathoms) and Echinochama, the 

 growth of the protoconch continues without material change until the valves 

 reach a length perhaps of two millimetres, the nepionic shell usually developing 

 concentric lamellae at regular intervals and having the interspaces smooth and 

 polished, or radially or concentrically striated. Having reached this size a 

 more or less sudden change takes place, and the adult sculpture is assumed. 

 Though the animal may not become sessile at once, it does so very shortly after 

 this new type of growth is exhibited ; in which state it was referred by Des- 

 hayes to the genus Cardita, to which, in a wide sense at this stage, it does 

 belong conchologically. With the sessile condition and the inability of the shell 

 to grow normally as before a kind of rotation results, and the ligament and 

 hinge-teeth are obliged to grow rapidly in a posterior direction, and the form of 

 the teeth becomes crude and obscure, being governed more by the dynamics of 

 its sessile state than by the hereditary model of hinge-teeth thus left behind 

 in growing. The dynamics of the hinge being practically the same whichever 

 valve is attached, the curious fact that in this genus the attached valve always 

 presents a similar hinge condition (whether the valve be right or left) is 

 accounted for. 



The hinge of the left valve of the protoconch of Echinochama at about two 

 millimetres length shows a ligament with no conspicuous nymph, a single large 

 cardinal slightly mesially grooved, and the rudiment of a second cardinal in 

 front of the large one near the dorsal border. In a specimen 4.5 millimetres 

 in length a callosity which may represent a third cardinal is developed on the 

 ventral side of the nymph, and is on its dorsal aspect distinctly crenulated. The 

 large middle cardinal has become relatively smaller and is now connected rather 

 obscurely with the anterior cardinal, which has elongated and become propor- 

 tionately larger, while below it on the margin of the hinge-plate a small corru- 

 gated thickening is perceptible. There is no trace of an anterior lateral at any 

 stage or in any species I have been able to study; if present, it has become 

 obscured by the marginal crenulations. The posterior lateral is, however, quite 

 distinct in most cases in both valves. In the right valve at this stage there are 

 two simple, subequal, diverging cardinals, but no callosity on the nymph. The 

 formula is L - 1 - 10101 -. As growth continues the teeth become tumid and corru- 



R. i.oioio. 



gated, more or less irregular within the limits of the species, but in a general 

 way the attached valve has a ventral and one or two dorsal corrugated ridges 



