MADREPORARIA APOROSA. 



267 



Another Palaeozoic type that may with great probability be included 

 among the TurbitiolidcB^ is the Silurian genus Duncanella. In this genus 

 (fig. 142) the coral- 

 lum is simple and 

 conical, with a deep 

 calice, and a well-de- 

 veloped wall marked 

 by costae and en- 

 circling striae. The 

 wall, however, is de- 

 ficient at the ex- 

 treme base of the 

 corallum, thus al- 

 lowing the primitive 

 septa to protude as 

 a little cone (fig. 

 142, b). There are 

 eighteen (sometimes 

 seventeen) septa, the | 

 symmetry of which 

 is completely radial, 

 and which meet cen- 

 trally so as to form 

 a sort of pseudo- 

 columella(fig. 142,0). 

 The interseptal lo- 

 culi, as in the Tur- 

 bi?ioZid<z generally, 

 are completely open, 

 and are free from en- 

 dothecal structures of any kind, though the bottom of the visceral cham- 

 ber is more or less extensively filled up by a deposit of stereoplasma. 



Lastly, it is probable that the imperfectly understood genera Petraia 

 and Polyccelia, both of which are of Palaeozoic age, may ultimately be 

 shown to be aberrant types of the Turbinolidce. 



Family 2. Oadinidce. — The corallum in this family is always 

 composite (fig. 143), the new corallites being usually produced by 

 lateral gemmation, and being united by an abundant and compact 

 ccenenchyma, the surface of which is smooth or striated, but is not 

 echinulate. The wall of the corallites is imperforate, not distinct 

 from the ccenenchyma, and the lower portion of the visceral cham- 

 ber usually becomes extensively filled up in process of growth by 

 a deposition of stereoplasma. The interseptal loculi are usually 

 open to the base, but dissepiments or (as in Lophohelid) tabulae are 

 occasionally developed. 



The Oculinidce appear for the first time in the Jurassic rocks 

 (Enhelia, Enallohelia, &c), and are also represented in the Creta- 

 ceous rocks (Synhelia, fig. 143, Diblasus, Baryhelia, &c.) In the 

 Eocene Tertiary we meet with early types of the Recent genus 

 Oculina itself, with its arborescent corallum and nearly smooth 

 ccenenchyma. The well - known living genera Amphihelia and 



Fig. 142. — Duncanella borealis, from the Silurian of North 

 America. A, Side-view of the corallum, enlarged twice ; b, Base of 

 the corallum, enlarged, showing the absence of the wall and the pro- 

 trusion of the septa ; c, Transverse section of the corallum, en- 

 larged nine times ; d, Vertical section of the corallum, enlarged four 

 times'. (Original.) 



