Proceedings, 11;") 



sponges, just as they did at the bottom of the cretiicaoiis 

 ocean. Among the chalk bivalve shells we find one class 

 very commonly, the Brachiopods. These differ very con- 

 siderably from the ordinary double-shelled Mollusca, and by 

 some naturalists are made into a separate group. The most 

 striking difference is, that, while in the ordinary Lamelli- 

 branch shells, such as Cockles and Oysters, the animal 

 wears its shells one on each side, the Brachiopods wear their 

 shells in front and behind, and may be compared with the 

 walking advertisements who bear two boards in the Loudon 

 streets. 



The number and variety of Sea Urchins which lived in the 

 Chalk Period is very noticeable, and, as different genera are* 

 found to characterise certain horizons in the chalk, we are 

 enabled by their means to classify the upper division into 

 zones. I have here a small specimen of Ananchytes ovatm, 

 from the Middle Chalk of Caterham Junction, which has been 

 taken possession of after the death of the animal and loss of 

 its spines by several forms of life, Ostrea, Crania, Bryozoa, 

 and Serpula. I spoke of the broken Inoceramus shells as 

 forming a large portion of the Lower Chalk. It is by means 

 of their different species that this lower division is classified 

 into zones. 



One of the most interesting problems is that afforded by 

 the Flint, and perhaps by the light of expeditions like that of 

 the ' Challenger ' we may obtain an inkling of its solution. 

 In the interesting paper read at our last meeting, Mr. Lovett 

 made an observation on the occurrence of sponges parasitic 

 on living marine animals, and alluded to some possible 

 bearing it might have on the phenomenon of fossils embedded 

 in flint. There is one division of sponges, the Clionida, 

 which bore into the shells of molluscs, and it is believed that 

 they accomplish this by means of their skeletons which are 

 enth'ely made up of siliceous spicules. The Clionida occur 

 fossil in very old formations, but little is as yet known about 

 them. At all events, many things seem to point to the pro- 

 bability that sponges had something to do with the formation 

 of flint, which has undoubtedly at some time or other been 



