284 ROBERT TRACY JACKSON ON THE 



shell which are referable in the closest manner to ancestral gi-on])s in eai'ly geologic times, 

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

 strikingly how the phylogenetic history of a group may be traced by the series of stages 

 presented in the early development 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 with Professor Lacaze-Duthier's observa- 

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

 are widely separated, though in the adult they almost come in contact, i-endering 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 

 aggregation into parallel lines of separate disc-like centres of calcification, each centre 

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

 which liave a byssus. These and other points of habits, anatomy and shell structure 

 ai-e discussed in sections xii and xiii. 



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

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

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

 cerned. Brief accounts are given of the young of sevei-al genera and a comparatively 

 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 prodissoconeh in Ostrea and its allies, Pecten, Perna, etc., I have 

 summed up in section xv the evidence of anatomy, shell structure and palaeontological 

 occurrence, all of which jioints towards Nucula or a nuculoid form as the probable an- 

 cestral type of which the prodissoconeh is the representative in the development of these 

 several genei-a. 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 ditfereut ages 

 of the individual and in related genera have gradually led. 



As a matter of technique I woidd call attention to the benefit received from the use of 

 drain-pipe trajis containing glass, as described in section u. 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 readere 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. SouBCEs OP Material, axd Methods of Woek. 



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

 it}^ on Buzzards Bay, which is situated on the southern shore of Cape Cod, fron\ which 

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



