Dr. F. A. Bather — The Lithodomom Worm Polydora. 109 



Norfolk" (1833), describes the denuded surface of the Chalk on 

 •which the Norwich Crag- was laid down. He refers particularly to 

 " a chalk-pit near Postwick church, the plateau of which, in its whole 

 extent, is perforated by an animal of the Tubicolaj family, as the rocks 

 of our present coasts are ". Specimens of this old sea-floor collected at 

 Postwick by Samuel Woodward himself show that the borings are 

 similar to those of Polydora, though rather coarser than those of 

 P. ciltata. In one specimen [A 275] the planed surface of the chalk 

 is clearly seen, but the borings are concentrated so thickly in places as 

 to have given rise to irregular cavities about a centimetre across ; in 

 another specimen [A 279] the chalk is so riddled that the planed 

 surface is no longer to be detected. From the same locality comes 

 a pebble formed from a Chalk fossil Echinocorys, and richly pierced 

 with similar tubes [A 274]. These borings must have been made 

 before the sea-shore was covered with the shell-beaches that now form 

 the Norwich Crag. They may, therefore, be regarded as of Plaisancian 

 rather than Astian age. 



Since these borings can only be expected in strictly littoral deposits, 

 such as are rarely preserved through a long period of geological time, 

 it is natural that traces of the genus should not have been observed in 

 the older Tertiary and the Mesozoic rocks of Britain. I cannot detect 

 them, as I had hoped, in our specimens of Ostrea lellovacina, or in any 

 of our Serpula-coxevedi Cretaceous and Jurassic shells, except perhaps 

 one. This is a Gryph($a incurva from the Sinemurian zone of 

 Arnioceras semicostatuni near Dorsington, Gloucestershire. [A 1183, 

 R. F. Tomes Colin.] The surface of both valves is covered with 

 numerous tubes of a small Serpula, and on the large valve are also 

 several oval pits each apparently leading to a double tube. The length 

 of the oval pit is only '4 or "omm., as compared with 1"5 to 2 mm. in 

 Polydora ciliata, and the burrows are perhaps more likely to be those 

 of a Dodecaceria. The specimen is merely mentioned here as affording 

 the nearest approach to Polydora burrows that I have been able to 

 find. All other specimens that are bored seem to have been attacked 

 by anything rather than Polydora. 



In view of this lack of evidence it was the more interesting to 

 receive from Mr. Linsdall Richardson an enquiry regarding a bored 

 pebble that he had extracted from the basal conglomerate of the 

 Rhaetic series at Hapsford Mill, Vallis, near Frome, Somerset. This 

 is a grey limestone pebble of irregular shape, with extreme measure- 

 ments 76 mm., 46 mm., and circa 50 mm. Its exposed surface is 

 almost entirely covered with borings which Mr. Richardson had already 

 recognized as similar to those in a pebble of chalk from the sea-shore 

 at Flaraborough. The worm that made these may well have been 

 a Polydora, but it was not P. ciliata, for the tubes are plainly larger 

 and their openings seem to have been further apart. The diameter of 

 the tubes in a recent specimen of P. ciliata is "5 to •6 mm. ; in the 

 Rha3tic fossil, -8 to '9 mm. Until the need arises for discriminating 

 between these Rhcetic borings and others that may be found in rocks 

 of approximately the same age, it scarcely seems advisable to invest 

 them with the dignity of a new specific name. 



"Whether as a disintegrator of rocks or as an index to the former 



