6l6 MOLLUSCOIDEA. 



being represented by numerous forms in all the great geological 

 formations from the Ordovician onwards. With a few possible ex- 

 ceptions, as already noted, the whole of the Palaeozoic Polyzoa may 

 be provisionally referred to the Cyclostomata : but while some of 

 even the oldest forms belong to quite normal groups of this sub- 

 order, others differ widely from the regular type, and will probably 

 be ultimately referred elsewhere. Leaving these last out of con- 

 sideration, the Cyclostomata may be said to attain their maximum 

 development in the Jurassic and Cretaceous rocks, a decided diminu- 

 tion in their numbers taking place in the Tertiary period. 



Taking the normal groups of the Cyclostomata first, and leaving 

 out of sight certain comparatively unimportant types, the family of 

 the CrisiidcB comprises a small number of dendroid forms, in which 

 the polyzoary consists of calcareous segments united by corneous 

 joints, the tubular zocecia being either uniserial or biserial (fig. 

 457, f). The type of this family is Crista itself, which is represented 

 by numerous Tertiary and recent species. 



The family of the Tnbulipo?'idce (Stomatopo?'idce) comprises forms 

 in which the polyzoary is usually creeping and wholly adherent by 

 its under surface, or partially free and erect. The tubular zocecia 

 may be disposed uniserially, or biserially, or irregularly grouped, 

 their upper ends being more or less extensively free. The ovicells 

 are represented by " an inflation of the zoarium at certain points, or 

 a modified cell " (Hincks). 



In the genus Tiibidip07'a itself the colony is more or less extensively 

 attached by its base (fig. 457, G), and the tubular zocecia are free for a 

 great part of their length and radiate from an excentric point. The 

 species of this genus range from the Cretaceous rocks to the present 

 day. The forms included under the head of Stomatopora {Alecto) differ 

 considerably in their characters. In one group of fonns comprised under 

 this generic name the colony is completely adherent to foreign bodies, 

 and the tubular cells are more or less extensively immersed, being free 

 close to their mouths only ; so that a more or less strap-shaped, often 

 branched frond is produced (figs. 451, A, and 459, B and c), the walls of 

 the zocecia being at the same time porous, and usually either biserial or 

 multiserial. In some forms (Proboscina), the polyzoary has the above 

 characters, but is partially free and erect. Forms of the type here in- 

 dicated are found in rocks as ancient as the Ordovician, while numerous 

 species still exist. On the other hand, there have been included in 

 Stomatopora various fonns in which the polyzoary consists of a creeping 

 adherent network, formed of uniserial or sometimes alternatingly biserial 

 cells, which spring directly from each other, each new tube being pro- 

 duced from the anterior end of the lower surface of the preceding cell 

 (figs. 458 and 459, d). The tubes are thick-walled and destitute of pores, 

 and the colony often closely resembles an Aulopoi-a in form. Indeed, the 

 only essential distinction between such colonies and Aulopoi'a is to be 

 found in the fact, that in the typical species of the latter there appears to 

 be a creeping basal network from which the tubes are sent up at inter- 

 vals, instead of springing directly from one another : but it is not clear 



