PHYLOGENY OF THE PELECYPODA. 379 



of the AviciilidjB and their allies. Nucula is dimyariaii, an adductor muscle existing at 

 either end of the equal valves. The gills are in two pairs, composed of simple, disumted 

 filaments' (Mitzixkuri) . The mantle borders are plain, without tentacles or a siphon, and 

 free around the margins. The foot is marked by a longitudinal cleft; accordingly it can 

 be flattened into a circular disc (Mitzukuri) and it has been observed crawling up the 

 sides of a glass of sea water (Tryon) indicating considerable clinging capacity of the 

 foot All these characters coinciding so closely with the features of anatomy and shell 

 structure known in the prodissoconch, together with the fact that ^ucula is known in 

 very early fossiliferous formations combine to render it extremely probable that Nucula 

 or a Nucnloid form is the type we are seeking as the early ancestral radical repre- 

 sented by the completed prodissoconch in the development of Avicula, Perna, Ostrea, 

 Pecten, Anomia and their allies. 



The fact that I^ucula is found in the Lower Silurian formations and still lives up to the 

 present day without sufficient changes in its shell structure to make these extremely 

 seijarable forms in time generically separable, argues strongly for a very high antiquity 

 for this genus. It was so firmly established in its earliest known forms that it must at 

 that time have been an ancient genus. It may therefore well have been the primitive 

 ancestral radical toward which the prodissoconch of the genera under consideration 

 points as the archetype, still indicated in the development of a modern individual. 



I have, therefore, from the results of these studies been able to trace, to Nucula or a 

 Nuculoid form as a probable type-ancestor, the prodissoconch which I have found to be 

 characteristic of developing Avicula, Perna, Ostrea, Pecten, Anomia and their allies. In 

 the next section (xvi) the relationship existing between these several genera and their 

 allies is traced as far as I am able to do so with my present understanding of the groups. 



XVI. Genealogical Connection of the AvicuLioiE and their Allies. 

 It is shown in section xv that the prodissoconch, which is characteristic of the Avic- 

 iilidse and their allies, as represented by typical genera, can be traced to Nucula or a 

 Nuculoid form as a probable ancestral radical of the whole group. In this section later 

 stao-es of shell growth are considered in their phylogenetic bearing and a tabular view is 

 given which represents the genealogical connection of the several genera as it is under- 

 stood from the results of these studies. Fcequent reference is made to the preceding 

 text for a full consideration of points only briefly alluded to, and many genera not pre- 

 viously studied are introduced. ^ -, j 



The earl V nepionic stage of Avicula, figs. 32-33, p. 330, Perna, PI. xxvii, fig. 1, and 

 Pecten PI.' xxviii, fig. 9, is closely alike in many species, indicating a common line of 

 ancestry for these genera. The points of resemblance are as follows:— The body of the 

 shell is sub-rhomboidal, oblique, the hinge line straight; the posterior wing is rounded 

 and is not produced beyond the margin of the body of the shell. Anteriorly the straight 

 hino-e extends beyond the limits of the prodissoconch, and is termuiated by a moderate 

 aurtcle as in Avicula and Pecten, or the auricle may descend directly from the uinbonal 

 area without any anterior extension of the hinge line as in Perna. In the several genera 



■Nucula 1U.S b.ou cou.idcred a primitive prototypal I'ol- pUcity of the gills, vvliich are paralleled iu the early stages 



ecypod by several investigators ou account of the sim- of many genera of I'elecypods. 



