[MATTHEW] STUDIES ON CAMBRIAN FAUNAS lOS 



all the species I Lave seen, is closed at the small end, this is effected by a 

 diaj^hragni, possibly the last of the series of diaphragms of a larval tube. 

 Serpula and Placostegus ^ exemplify best the thick calcareous tubes of 

 some Hyolithes ; and the former in its operculum best represents that of 

 Orthotheca ; from the operculum of Orthotheca that of Serpula differs in 

 the greater number of rays which mark it. Pel^neer as above remarked, 

 says the operculum of Hyolithes is not that of an oi)erculate Mollusc and 

 this perhaps because it does not show the spiral growth seen in those of 

 Gasteropods, but exhibits a radiate arrangement of parts. Fomatoceras 

 strigiceps ' and Serpula nacronensis ^ both show these radiations in the 

 operculum, and have thick calcareous tubes, somewhat pentagonal in form 

 like some species of Hyolithes. 



Since then we may no longer associate the Hyolithidae with Ptero 

 poda, a place may be found for them among the Tubicolous Worms. Theo- 

 retically we may regard these tubes as having a larval initial portion, per- 

 sistent in soa:e species but caducous in most. This tube was very slender, 

 cylindrical, and in some cases hyaline (therefore probably very thin). 

 This larval tube passed into one of calcareous substance and conical, which 

 possessed one or more diaphragms or septa. At this point a Hyolithes 

 tube is apt to be decollated so that usually the larval portion is wanting. 

 Beyond the septa, in the upper part of the tube, we can usually find two 

 conditions of structure in the shell, a lower part, firmer and more rigid 

 owing to a thickening of the test which is here strengthened by vertical 

 rods within its substance ; and an upper part where these rods are want- 

 ing, and the shell is thinner ; this part of the tube has usually not so wide 

 an apical angle as the camerated part below. 



Linking the Hyolithidfe with the Worms are two families distin- 

 guished by their shell substance, but otherwise having much in comm'on 

 with the above family. One of these (defined by Di-. Gerhard Holm 

 in his notable essay on the Swedish Cambrian -Silurian Hyolithidae and 

 Conulariida?) is Torellellidœ, founded on the genus Torellella, having a 

 phosphatic test ; the other is that to which the genera Urotheca and 

 B3'ronia belong, genera which are described in connection with the fauna 

 of Mount Stephen in a preceding article ; in these the tube is chitinous. 

 No larval tube has been described for Torellella, but we find it distinctly 

 present in the species of Urotheca of the Mount Stephen Fauna. These 

 two families differ from Hyolithidœ in having a flexuous tube. Hyolithes 

 while often curved is bent in one plane, but both Urotheca and Byronia 

 as well as Torellella are often bent in more than one direction, and by 

 their flexible and camerated tubes show close affinity with the sea worms. 



' Chall. Rep., vol. xii., p. 524, pi. .55, fig. 7. 



- Chall. Rep., vol. xii., p. 522, pi. 55, figs. 5 and 6. 



3 Ibid. p. 516, pi. 55, fig. 1 and p. 54, fig. 5, 



