78 rilOCKEDINGS OF THE MALACOLOGICAL SOCIETY. 



followed hy Helminthochiton, Chelodea, etc., in the Silurian, Proholceum 

 in the Devonian, Gryphochiton in the Carboniferous, and so on. For 

 these Palaeozoic Chitons Pilsbry (in Zittel, 61, p. 434) has established 

 the family Gryphochitonidae, characterized by the absence of insertion 

 plates. The modern Lepidopleuridae, which date from Tertiary times, 

 are closely allied to them, and, though the higher genera of this 

 familj^ possess insertion plates, these are still unslit. This antique 

 type has only been able to exist to the present time by taking to 

 deeper water, where competition in the life-struggle is less severe 

 (Pilsbry, 44, p. x). 



That older representatives of the Polyplacophora will yet be found, 

 I feel sure, and would suggest search for them among fossils referred 

 to the plates of Pod-Shrimps, seeing that one at least has thence been 

 brought to light (H. Woodward, 57, pp. 356-358). 



The more highly specialized Aplacophora have not been recorded 

 fossil as yet, but some day an energetic microscopist, if any such be 

 left, may when hunting over fossil sponge-spicules come on examples 

 of those of these interesting molluscs. 



Measured, then, against the record of the rocks as at present revealed 

 to us, the story of mollusean development according to the latest 

 morphological investigations, based on the study of living forms, is 

 not in complete harmony therewith, although much more so than one 

 would at first be led to suspect. The Amphineura, Scaphopoda, and 

 Opisthobranchia all make a relatively more tardy appearance in the 

 strata than they theoretically should do, while the early Gastropods 

 have distinctly not sorted themselves out phylogenetically. This 

 circumstance is due largely no doubt to the imperfections in the 

 geological record, largelj^ also to the fact that the true relationships of 

 those fossil forms that are known to us, especially the older ones, are 

 not yet satisfactorily determined. Chiefly is this the case Avith the 

 Gastropoda, in which class, as Eastman remarks {61, p. 502, note), 

 " the difficulty of adapting a strictly zoological classification, based 

 upon the anatomy of the soft parts, to the practical needs of the 

 palaeontologist is strikingly illustrated." 



Although this difficulty may never be entirely overcome, many of 

 the gaps in our knowledge may be successfully bridged by steady 

 patient work of the kind alreadj^ attempted by Hyatt, Buckman, 

 Waagen, and others, for the Ammonites; by Cossmann, Grabau, and 

 others, for the Gastropods; and by Jackson for the Pelecypods; the 

 method adopted being to trace out the relationship and succession 

 of the various allied species in a given group, or, better still, to trace 

 the morphology of a given form both laterally along a definite horizon, 

 and vertically into successively newer and newer beds. It had been 

 my intention to allude more fully to this method of working, and 

 the class of results it yields, but the subject is one that will well bear 

 treatment at greater length on some future occasion when more time 

 can be devoted to it. 



The lacunae in our knowledge of the interrelationships of the 

 members of the various families and orders of MoUusca are slight, 

 however, compared with the blank caused by the total absence from 



