PTEROPODA. 



805 



a left-handed spiral, and a spiral operculum is present. The anus is 

 on the right side, and the pallial cavity is dorsal in position. It is 

 not certain that any fossil forms of this family have been detected, 

 but some minute shells from the Eocene, Miocene, and Pliocene 

 deposits have been referred with considerable probability to the 

 existing genus Limacina (Spinalis). 



Family 2. Cavoliniid^e. — In this family the shell is bilaterally 

 symmetrical and is not rolled up into a spiral. The anus is on the 

 left side, and the pallial cavity is ventral. The shell has a variable 

 form, but is essentially a hollow cone, which may be flattened dorso- 

 ventrally or may be circular in section. The initial portion of the 

 shell is generally distinct from the rest, and represents the larval 

 shell (Pelseneer). 



The types which have been described under the names Creseis, 

 Hyalocylix, Clio (sensu restricto), and Styliola, are included by Dr 

 Pelseneer under the common generic name of Clio ( = Cleodora). 

 In all these forms the shell (fig. 722, c) is a conical tube, sometimes 

 circular in outline, sometimes oval, sometimes laterally keeled, the 

 apex of the shell being pointed, or exhibiting a bulbiform enlarge- 

 ment due to the presence of a distinctly defined embryonic shell. 

 Unquestionable remains of this group of Pteropods are found in the 

 Miocene and Pliocene deposits. 



In the Silurian and Devonian rocks of both the Old and New 

 Worlds there occur minute conical calcareous tubes, which have been 

 generally referred to Styliola or Creseis. 

 As the type of these may be taken Styliola 

 {Creseis) fissurella of the Devonian rocks of 

 North America. In this remarkable form 

 (fig. 719) the shell is a very delicate cal- 

 careous tube, of a conical form, circular 

 in section, and without internal partitions, 

 either longitudinal or transverse, the aver- 

 age length of the tubes being from one to 

 three millimetres. The apex of the tube 

 is commonly slightly bulbous (fig. 719, a), 

 and is thus marked or! as an " initial 

 chamber " from the rest of the shell. The 

 surface is without annulations, and may be 

 smooth, or marked by fine transverse striae, 

 sometimes with longitudinal striae as well 

 (fig. 719, b). Styliola fissurella is exceed- 

 ingly abundant in parts of the Devonian 



rocks of North America, and sometimes gives rise by the ac- 

 cumulation of its shells to thin bands of limestone (fig. 720), 

 which may have a considerable geographical range. Dr Pelseneer 



Fig. 719. — A and b, Specimens 

 of Styliola {Creseis) fissurella, 

 Hall, from the Devonian rocks 

 (Hamilton Group) of North 

 America, enlarged about six 

 times, showing the embryonic 

 shell at the apex ; c, Apical por- 

 tion of the tube of Tentaculites 

 gracilistriatus, from the same 

 formation, enlarged to show the 

 embryonic shell. (After Hall.) 



