102 BULLETIN OF THE 



elements of structure found in subsequent forms besides their greater 

 simplicity, but this can be accounted for best by considering them as 

 forms having a truly retrogressive development, what has usually been 

 described as a retrograde metamorphosis. 



The types of Ascoceras which I have seen are much too imperfectly 

 preserved to enable me to speak from my own knowledge of the nicer 

 points of structure described by Barrande, but that author's unsur- 

 passed and minutely accurate figures supply all deficiencies. One 

 peculiarity of the structui'e appears to be very unfavorable to the view 

 here presented and is so considered even by the author himself; this is 

 the want of connection between the young siphon and the supposed 

 larger siphon of the adult. The first of the imperfect septa bends 

 posteriorly, as do the others, but is not, however, discontinued when it 

 reaches the last of the entire septa of the young. It is prolonged over 

 the surface, and is really a complete septum, not pierced by any si- 

 phonal opening. We cannot imagine any normal progressive mode of 

 growth by which the minute so-called siphon of the young Ascoceras 

 could be changed into the huge visceral prolongation of the living 

 chamber of the adult, without some of the intermediate steps of this 

 change being visible, and some connection maintained with the siphon 

 of the young. Barrande also states that in no instance has he found 

 any more than one of the perfect elliptical septa of the young pre- 

 served ; that over this he has observed the striations and markings of 

 the external shell ; and further, that the so-called minute siphon, or 

 elliptical funnel, penetrates this septum. Now this condition is precisely 

 what we should expect to find, if a portion of the exposed lower end 

 of the Ascoceras, or area of the first septum represented the scar left 

 by the ovisac on the apex of the whorl, as in Nautilius Pompilius. 

 This would not only account readily for the presence of the strirc of 

 the shell upon the exterior, but also for the projecting end of the so- 

 called siphon. The latter would then represent the siphonal coecum, 

 which, as in Goniatites and Ammonites, penetrated the first septum, 

 and by the removal of the ovisac had been left exposed. 



Of course this explanation can only be considered as a suggestion 

 calling for a re-examination of some of Barrande's fine specimens, 

 and is quite as likely to be overthrown as to be confirmed by such a 

 process. 



Whether this be so or not, the young Ascoceras was evidently, as 



