PROSOBRANCHIATA. 783 



this family are marine, and the two principal types are Vermetus and 

 Siliqnaria. The genus Vermetus is remarkable for the resemblance 

 of its tubular shell to the calcareous tubes of Serpula. The shell is 

 usually attached to some foreign body, and though regularly spiral 

 when young, is always irregular in its growth when adult. The fossil 

 species are best distinguished from Serpula by the fact that the tube 

 is repeatedly partitioned off by calcareous septa as the animal grows. 

 It is, however, often a matter of extreme difficulty to determine 

 whether a given specimen be a Vermetus or a Serpula. The range 

 of Verjjietus in time, using this name in its wide sense, is from the 

 Carboniferous period to the present day ; but most of the fossil 

 forms belong to the later Secondary and Tertiary deposits. In the 



Fig. 682. — Turritella angulata. Fig. 683. — Siliquaria anguina. 



Neocomian. Pliocene and Recent. 



genus Siliquaria (fig. 683) the shell resembles that of Vermetus in 

 most respects, but it is free, and the tube has a continuous longi- 

 tudinal slit, which may be replaced by a row of pores, running along 

 the whole length of the shell on one side. The genus ranges from 

 the Cretaceous to the present day. 



Family 25. Cjecibm. — This family comprises only the genus 

 Ccecum, in which the shell is free, and, to begin with, forms a small 

 flat spiral. With age, however, the shell becomes decollated, and 

 has in its adult state the form of a curved cylindrical tube, the 

 truncated end of which is closed by a transverse calcareous partition. 

 A number of recent and a few fossil forms of the genus are known, 

 the oldest of the latter being found in the Eocene Tertiary. 



Family 26. Pyramidellid^e. — In this family the shell is spiral 

 and turreted, often with a polished surface. The aperture is small 

 and entire, sometimes with one or more prominent plaits on the 

 columella. The operculum is horny, ear-shaped, and paucispiral. 

 All the members of this family are marine, and most of the living 

 forms are of small size. 



