284 ROBERT TRACY JACKSON ON THE 



shell which are referable m the closest manner to ancesti'al groups in eai-ly geologic times, 

 from which the Pectens were doubtless evolved. The life-history of Pecten shows most 

 strikingl}^ how the phylogenetic history of a group may be traced by the series of stages 

 presented in the early develojiment of the individual (sections ix-x). 



Studies were made of young Anomias (PI. xxix) growing on glass slides and yielded 

 facts of high interest', especially in connection Avith Professor Lacazc-Duthier's observa- 

 tions on the adult. In the young the three muscles, adductoi-, posterior pedal and byssal, 

 are widely separated, though in the adult they almost come in contact, rendering their 

 nature ambiguous. The byssus of Anomia was seen to originate from a cleft in the foot 

 and in the young consists of separate threads. The calcareous plug is formed by the 

 aggiegation into parallel lines of separate disc-like centi-es of calcification, each centre 

 being comparable to a separate byssal-thread attachment found in ordinary Pelecypoda 

 which have a byssus. These and other jwints of habits, anatomy and shell structure 

 are discussed in sections xn and xin. 



The section xiv, entitled Studies of a few other Genera, considers the young of Mytilus, 

 Argina, Venus, Mya, etc., and is intended principally to show the extension of the pro- 

 dissoconch to other genera besides those with which this paper is moi-e especially con- 

 cerned. Brief accounts are given of the young of several genera and a comjsaratively 

 extended description of the development of the siphon and byssal attachment in Mya. 

 The considerations of this section are for the most part of a preliminary nature and it 

 is ho])ed to give a full description of these and other genera in a future publication. 



After studying the j^rodissoconch in Ostrea and its allies, Pecten, Perna, etc., I have 

 summed up in section xv the evidence of anatomy, shell structure and pala^ontological 

 occuiTcnce, all of which points towards N^ucula or a nuculoid form as the probable an- 

 cestral type-of which the prodissoconch is the representative in the development of these 

 several genera. Passing to the next section (xvi),I have attempted to give in tabulated 

 form the serial relations existing between the genera studied. These two sections may 

 be considered as the culmination, to which the studies of shell growth at diflerent ages 

 of the individual and in related genera have gradiially led. 



As a matter of technirpie I would call attention to the benefit received from the use of 

 drain-pipe traps containing glass, as described in section n. By its means attached 

 forms were studied on a transparent medium and free forms of small size were entrapped 

 and easily found. 



For the guidance of readei-s attention is called to the italicized paragraphs on pages 295 

 and 300. Every care has been used to give full references to all observations bori'owed 

 from authors and equal pains have been taken to give full credit to investigators where 

 their observations seemed similar to mine, or to have bearings on the various facts and 

 conclusions discussed. The facts described and inferences from them not specially cred- 

 ited in this paper are my own. 



II. SouECEs OF Material axd Methods of Work. 



In the summers of 1887 and 1888, several weeks were spent at a fine collecting local- 

 ity on Buzzards Bay, which is situated on the southern shore of Cape Cod, from which 

 I walked, or rowed, to my laboratory not far off. 



