378 ROBERT TRACY JACKSON ON THE 



per. These two lines of iiujuiry linked with the i)roper paheontological sequence are 

 found to point directly to a form which fills the necessaiy requirements of such an ances- 

 tor. Data for the inquiry. Tlie shell. In Avicula, Perna, Ostrea, Peeten, Anomia 

 and allies a prodissoconch exists in which the structure is homogeneous and laminar, 

 sub-nacreous, not prismatic, though succeeded by a dissoconch having well-defined pris- 

 matic structui'e. The umbos of the prodissoconchs are directed posteriorly, and the 

 hinge line is curved. The only genus in which we know the prodissoconch before any dis- 

 soconch growth has taken place, so to enable us to determine features of the hinge line, 

 at this stage is Ostrea. In Ostrea edulis, fig. 2G, p. 312, the prodissoconch has teeth 

 at right angles to the hinge line, and these teeth closely resemble the teeth of Nucula, 

 when a shell of that genus is viewed from the umbonal region as in the figure cited. In 

 0. virgmiana, however, no teeth were observed. The anatomy as illustrated by the va- 

 rious genera yields points of great synthetic value. The prodissoconch has two ad- 

 ductor muscles as proven in Osti'ea, PI. xxiv, figs. 1-2, and inferred in the other genera 

 from the form of the prodissoconch valves and the kinship to Ostrea (see studies of 

 Perna, Avicula, Peeten and Anomia). In the prodissoconch stage of Ostrea the gills 

 are simple unpaired filaments and they are evidently simple filaments at first in other 

 s:enera as indicated in our studies of Anomia and Peeten.^ Tlie mantle border of the 

 ])rodissoconch is free around its entire margins except at the hinge line in Ostrea, and 

 bears no tentacles or siphons. A similar condition of freedom exists in the youngest 

 stages we know of related genera, and the development of tentacles and eyes described 

 in Peeten indicates that at a very eaily period the border was without them in that ge- 

 nus. A foot evidently exists in the t3^pical prodissoconch, for though wanting in Os- 

 trea," it is present and highly developed at the close of the prodissoconch period in the 

 young of many related genera (Avicula, Perna, Peeten, Anomia, etc.), as I have shown. 

 The foot where known is marked by a proximal byssal cleft and a distal cleft which lat- 

 ter is flattened out when crawling, giving the foot a disc-like, clinging power (see Pec- 

 ten and Anomia, sections ix and xii). 



With the above described jrwdissoconch features as Tcnoimi characters we have to seeTc 

 for possible kinship ivith ancient forms. These characters are found to be combined in 

 a remarkable degi-ee in the ancient genus IsTucula which, though living to-day, extends 

 back to very early fossiliferous formations, fifty species (Tryon) being known from the 

 palaeozoic. Nucula is one of the few genera known which has the umbos of the shell 

 directed toward the posterior end of the body.^ The shell of I^ucula is oval, nacreous, 

 not prismatic' and has a curved hinge line with teeth at right angles to its axis. A tri- 

 angular, central cartilage i)it is a characteristic feature of the Nuculid;^, and though it 

 is not definitely known to exist in the prodissoconch it may exist at that stage in some 

 irenera, and is a marked feature of the succeeding dissoconch in all the ascendent series 



' A simple, filamentous condition of the gills as first de- bos are directed posteriorly as far as I have found from 



scribed by Lacaze-Duthiers in Mylilus is considered l)y the conchological manuals, are Anatina, Donax and Tel- 



cnibryologists as characteristic of all developing Pelecy- lina, the two latter belonging to one family, 



poda. * Traces of prismatic cellular structure are noted in Nu- 



'The absence of the foot in Ostrea is. fully discussed in cula by Carpenter, but he equally notes It in many other 



previous jKiges. genera of Pelecyi)ods, as sometimes seen in slight patches 



^ The only genera outside the Nuculida;, in which the um- though not a feature of the shell as in Avicula, etc. 



