98 A. E. Verrill — Molluscan Archetype, 



rocks, certain small shells are found which appear to be true 

 heteropods.* 



It is also easy to believe that adult free-swimming bivalves, 

 with a permanent velum, may have been abundant in the 

 ancient seas. It is even possible that some of the described 

 small and delicate Cambrian bivalves may have been thus fur- 

 nished. It is not difficult to understand how such forms may 

 have developed a foot suitable for creeping or for adhesion by 

 a sucker or byssus while still retaining the velum, for this still 

 occurs in modern veligers (figs. 11, 19). It is significant that 

 the most primitive of living bivalves {Nucula^ Leda^ Solemya^ 

 etc.) have the. foot so modified that it can be used as a very 

 effectual swimming organ. The allied ancient forms may have 

 had a similar foot without losing the velum, and thus there may 

 have been strictly pelagic adult bivalves of considerable size. 



The primitive Cephalopoda were probably also derived from 

 veliger-like forms, though no true cephalopod veligers are yet 

 known. It is not difficult to understaud how a cephalopod 

 could be derived from a proveliger similar to that of some 

 pteropods and gastropods, by a special development of the foot 

 into prehensile processes around the enlarged head, while the 

 gill-cavity developed most on the ventral side, as in some 

 pteropods. On the other hand, it seems impossible to derive 

 a cephalopod or a bivalve from a creeping chiton-like archetype, 

 such as Lancaster has proposed. 



* Especially the genera Microceras and Cydura of the Lower Silurian. Some 

 species referred to the former genus, which I have examined, have a dorsal carina 

 and median slit like species of the modern genus Atlanta, which they also resem- 

 ble closely in size and form. 



EXPLANATION OF FIGURES 4-15. 



FiauRES 4-11. — Larval gastropods; 4, 5, younger and older veliger stages, 

 diagrammatic ; 6, veliger of anudibranch (Polycera) with shell; 7, simple incurved 

 stage of early veliger shell of undetermined species ; 8, early veHger of Hmpet 

 (Patella) with similar subspiral shell ; 9, mature veliger of Triforis, with reversed 

 shell and elongated foot; tentacles and eyes are well formed; 10, veligers of 

 Natica, b, with four-lobed velum expanded ; c, d, with velum nearly retracted, 

 but showing foot; 11, the same, more enlarged and somewhat older, in the act of 

 crawling, showing a well developed spiral shell, s, and a broad creeping foot, /, /, 

 while the four-lobed velum, v, v, is still functionally active and capable of being 

 used for swimming (from life). 



Figures 12-15. — Larval pteropods (after Fol); 12, proveliger of Cavolina 



13, subveiiger of Cavolina; 14, veliger of the same; 15, more advanced young; 

 to show curvature of shell probably due to action of the retractor muscle, r, r'. 



The lettering is the same for all the figures of veligers in this paper (figs. 4-19); 

 op, apical plate; an, anterior pole; a, mouth; 6, oesophagus; c, stomach; d, 

 intestine; e, anus; e', its rudiment; /, foot; /', epipodium or wings of pteropod ; 

 g, gill-cavity ; g% gill ; /i, heart ; h\ aorta ; i, eye ; k, nephridium ; k', its duct ; 

 Z, liver; l\ material out of which it is formed; m, mantle; m% mantle edge; n, 

 cerebral ganglion; n\ pleural ganglion; n\ pedal ganglion; o, operculum; ot, 

 otocyst; r, r', retractor muscles of velum; s, shell, s', shell gland; s", its inte- 

 rior ; t, tentacle ; tt, u, nutritive sacs, or yolk substance ; -y, velum ; x, anal cells. 

 The short vertical lines at each end indicate the antero -posterior axes of several. 

 All are much enlarged. 



The sources of the veliger cuts are as follows: 4, 5, from Gegenbauer; 6, from 

 Lancaster; 7, 9, 10, 11, from nature, by J. H. Emerton ; 8, after Patten; 12, 13, 



14, 15, after Fol; 16, after Kowalevsky; 17, 18, after Hatschek; 19, after 

 Korschelt and Heider. 



