Oumings — Development of some Paleozoic Bryozoa. 61 



such a position as to give a backward trend or deflection to 

 subsequent budding, thereby giving rise to a hippocrepian (fig. 

 46) and later to a circular aggregation of buds (initial circle, 

 metanepiastic stage, figs. 47-51) in which the protoecium occu- 

 pies one end of a diameter, and the last formed bud of the 

 circle, the other. The median primary bud arises from the top 

 of the protoecium considerably later than the lateral buds, i. e., 

 at about the time of completion of the initial circle. It forms 

 the first individual of the second tier of buds, which lie sym- 

 metrically above the first tier. 



Metanepiastic Stage. — In Fenestella this stage consists of 

 the completed initial circle of zooecia. It has been seen that 

 the ananepiastic stage trespasses on it to the extent that the 

 median bud does not arise till near the completion of this 

 initial circle. It constitutes an incrusting growth formed by 

 budding in a horizontal plane, from which the adult colony 

 arises by budding in vertical planes. Typically it consists of 

 ten zooecia including the protoecium, but there is considerable 

 variation from this number even within the limits of a single 

 species, so that initial circles of eight, twelve, or fourteen 

 zooecia are not uncommon, while in a peculiarly accelerated 

 form found at Thedford, Ontario, only five of the typical 

 number develop. In the latter, circles containing ten zooecia 

 arise only after several generations of zooecia have been pro- 

 duced in vertical series. Such a zoarium is shown in fig. 55. 

 The full discussion of this type of zoaria is deferred to a later 

 time. 



In certain cases the initial circle is arrested at a hippocrepian 

 or semicircular stage, either because the full number of zooecia 

 fails to develop, or because the trend of the budding produces 

 too great a divergence of the series of buds to the right and 

 left of the protoecium. From a metanepiasty of this type, a 

 flabellate zoarium will arise, unless subsequent growth is of such 

 a nature as to bring the lateral margins of the zoarium together. 



Description of Figures 47-54. 



Figures 47-54. — Early stages in the development of Fenestella from the 

 Hamilton limestone of Canandaigua Lake, N. Y. (47-50, 53-54), and the 

 Lower Helderberg limestone of Indian Ladder, N. Y. (51, 52). 



47-50. Top and side views of a young Fenestella slightly older than the 

 individual shown in fig. 46. Initial circle of zooecia complete and walls of 

 neighboring zooecia fused more completely than in 46. This specimen has 

 ten (the typical number) zooecia. The origin of the primarv carinas is shown 

 at a to/. 51. Another specimen at about the same stage as 47, primary 

 carinae a to d. Each of the pits, a' to d" ', corresponds to two zooecia ; a faint 

 septum may be seen bisecting a" . The depressions in the basal plate (1 to 8) 

 mark the position and limits of the exserted polypides. 52. Stage somewhat 

 older than 51, showing first branching of primary carina (di, c? 2 ) and three 

 tiers of zooecia (z, z, z, z, z, z). Note the great size and prominence of the 

 carinas. 53, 54. Another individual of the same age as 47. 



All figures x 40. 



