Cambridge Philosophical Society. 503 



appears destitute of the longitudinal striae which Rudolphi and Du- 

 jardin describe as distinguishing the S. equinutn. The circular rings 

 on the centre of the body are about the fourth (but nearer the head 

 only a third) of a millimetre apart from each other ; whilst in the 

 ordinary specimens of 5'. equinum they are described as being very fine, 

 and according to Dujardin only 0'0043 mill. The tail, too, is much 

 more obtuse than in the ordinary specimens of the S. equinum, and the 

 whole animal is more robust, though not longer. The specimen 

 appears to be an immature individual ; and as I have not been able 

 to refer to Gurlt's notice of the specimens he found in the tunica 

 vaginalis of the testicle of the horse, I must satisfy myself at present 

 with merely recording the fact of its being found in this country 

 also, trusting to have an opportunity at some future time of exami- 

 ning additional specimens. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 

 ON THE FEN CLAY FORMATION. 



The following notice is a brief abstract of a paper read by Mr. Harry 

 Seeley at the annual general meeting of the Society, on Monday, 

 October 28th. 



Extending under the peat of the fen district, and far beyond, is a 

 great clay formation. It includes the Oxford and Kimmeridge clays, 

 and an intervening clay (replacing the Coral-rag) which imperce})ti- 

 bly graduates upwards and downwards into those deposits. It is for 

 this series of strata, ranging from the Great Oolite up to the Port- 

 landian beds, that the term Fen-clay or Fen-formation is proposed. 

 The fact of such a succession in some degree interferes with existing 

 views of the division of the lower secondary strata into Upper, 

 Middle, and Lower Oolites ; so that henceforth it will probably be 

 found more convenient to abandon those terms, and to speak of the 

 secondary formations below the Cretaceous series as Lias, Inferior 

 Oolite, Great Oolite, Fen-clay, and Portland beds. In this district 

 the Fen-clay extends from the line of Peterborough to Bedford, across 

 easterly to the line of Ely and Lynn, within which limits it has been 

 chiefly studied, though known to have an extensive development 

 further south. 



The various subdivisions were worked out in the country around 

 Elsworth, near St. Ives. That village is built on a limestone to 

 which it gives a name, the Elsworth Rock, which consists of three 

 subdivisions, an upper and lower rock, and a middle clay which 

 abounds in Ostrcea Marshii. The rock dips to the south, and main- 

 tains its thickness (14 feet) unchanged for the three miles over which 

 it could be traced, though at that distance the middle clay is replaced 

 by sandstone. 



Passing to the north, another rock is met with, at St. Ives ; and 

 this was shown to be 130 feet below the Elsworth rock, coming out 

 from under it, being brought up by an anticlinal axis, so that further 

 to the north, at Bluntisham, the Elsworth rock is again met with. The 



