242 W. G. RIDEWOOD. 



twenty pairs of pinnules^ and the middle ones of the series 

 are the longest. 



Development of the Plumes in Buds op Cephalodiscus 



gilchristi. 



Cephalodiscus gilchristi resembles Cephalodiscus 

 nigrescens, and differs from Cephalodiscus dodecalo- 

 phus and Cephalodiscus hodgsoni, in the tardy deve- 

 lopment of the plumes in comparison with the rapid growth 

 of the buccal shield. The bud, at first pear-shaped (text-fig. 

 9, a), soon becomes differentiated into a buccal shield and a 

 " bod}'/' with stalk (b). Even at this early stage of develop- 

 ment the posterior lobe of the buccal shield is well defined 

 {b, p.L). A pair of mounds appear on the right and left 

 sides of the "body," near its junction with the middle of the 

 shield (c and d), and these develop into the first pair of 

 plumes. It is to be noted that these mounds are strictly 

 lateral structures, and do not project forward as do the first 

 plumes of the other three species under consideration during 

 the early stages of growth. 



The second pair of plumes arise postero-ventrally to the 

 bases of the first (e and f, 2), and in a dorsal view such as 

 that shown are partially concealed by the first pair. When 

 the third plumes begin to develop (g, 3), the first pair project 

 autero-laterally instead of strictly laterally, possibly owing 

 to the pressure put upon them by the second and third 

 plumes, which arise posteriorly to their bases. Each of the 

 six plumes now present is wider at the middle than at its 

 base. 



When the fourth plumes make their appearance (h, 4) the 

 length of the first and second plumes is more than twice 

 their width at the base ; and before there is any sign of the 

 fifth plumes pinnules begin to differentiate upon the first two 

 pairs of plumes (j, 1 and 2). The first plumes are now 

 directed more anteriorly than laterally, and the post-oral 

 lamella is clearly distinguishable (j, po.L). 



