ON THE DEVELOPMENT OF TUBULIPORA. 79 



expansion begins the ovicell has become separated by younger 

 zooecia from the basal lamina, and its floor is thus formed by 

 their proximal parts. The upper part or roof of the ovicell 

 spreads out horizontally as the conspicuous, porous, calcareous 

 film usually described as the ovicell. If no fresh zooecia were 

 formed at the growing edge, the roof of the ovicell would be a 

 simple fan-shaped film. But new zooecia continue to grow in 

 the same way as if no ovicell were present (PI. 10, fig. 32). 

 The floor of the ovicell, formed by the upper walls of the 

 younger zooecia, or really by the septa dividing its own body- 

 cavity from that of these zooecia, thus rises into radial ridges 

 which encroach on its cavity. As these become more vertical 

 they meet the growing horizontal roof of the ovicell, and with 

 further prolongation stand up from it at a right or obtuse 

 angle. 



In most of the species investigated the zooecia which are 

 younger than the ovicell are arranged in connate series. But 

 whereas the proximal parts of the fertile lobes have their 

 zooecia in obliquely transverse, alternate rows (fig. 5), these 

 series usually become radial in the region of the ovicell. In 

 other words, a young zooecium which has reached the roof of 

 the ovicell in the manner just indicated does not become free, 

 but remains connected with the expanding growing edge by 

 a still younger zooecium. A radial series of connate zooecia 

 thus results, and the roof of the ovicell is divided into two 

 lobes, one on each side of the zooecial series. After a time the 

 series may become completely free from the growing edge, and 

 the two adjacent lobes of the roof of the ovicell may then unite 

 distally to the series. 



In my preliminary note (17, p. 212) I have stated that the 

 zooecia may thus be left as columns passing freely through the 

 cavity of the ovicell. I believe that this statement is not 

 correct for T u b u 1 i p o r a. The cavity of the ovicell may indeed 

 surround the zooecium, or series of zooecia, on the proximal, 

 lateral, and to some extent on the distal sides; but I have 

 found no evidence that two lobes which have become con- 

 tiguous on the distal side of a zooecium ever really unite, as 



