302 G. C. Rohson—On Helminthochiton. 



prominence in this region of the compass is met with in any other 

 Echinoid, fossil or recent — in fact, the usual articulation for the 

 circular muscle is rather a depression than an elevation. The compass 

 of Pholidechinus (Jackson, loc. cit., pi. xxvii, fig. 5) shows nothing 

 comparable, but is not verj'^ perfectly preserved. That of Arckceoa'daris 

 rossica (loc. cit., pi. xii, tigs. 1, 2) is particularly slender and smooth 

 in outline. 



The precise function of the circular muscle is not known. It is 

 therefore idle to speculate as to the reasons for this extraordinary 

 development in the compass of PeriscJiodomus. It suffices to record 

 that it is unique among known forms. 



Tooth. — A small fragment of the distal end of a tooth is shown on 

 the specimen. It is 58 mm. long (as far as exposed), and 4"25 mm. 

 broad before it begins to taper. It is, as usual, Aulodont in character, 

 and the groove is shallow. The point is broken away. 



The specimen is now in the Manchester Museum, registered 

 No. L. 8722. 



V. — Helminthochiton ^quivooa, n.sp., Lowkr Ordovician, 

 Bohemia. 



By Guy C. Eobson, B.A. 

 (Published by permission of the Trustees of the British Museum.) 



^MHE species described in this paper is based on specimens acquired 

 J_ for the Geological Department of the British Museum in 1912 

 from Professor C. Kloucek, of Prague, ^who collected them from the 

 upper portion of D^^ ( = Arenigian) at Harka and Male Prilepy. The 

 fossil was referred in the first instance by Professor Kloucek to 

 " Chiton sp. '?" and the result of the present investigation has been to 

 uphold the reference to the Polyplacophora, though the precise generic 

 status must remain vague. 



The remains themselves consist of a number of imprints of shell- 

 fragments in portions of two separate ironstone nodules, and have 

 been studied by means of gutta-percha squeezes. In the nodules the 

 imprints are arranged in a crude linear series of indeterminate form, 

 while several of them lie apart from the main series. Two only have 

 the appearance of being applied to one another in the characteristic 

 Chiton fashion. A large number of the fragments are too amorphous 

 to justify the expenditure of time in studying them. We therefore 

 direct attention to six pieces, the shape and texture of which are 

 sufficiently well characterized to enable one to form an opinion of 

 their nature. These, however, are in no case connected with each 

 other, nor do they afford any grounds, save those of general 

 resemblance, for referring them to the same individual. 



A typical fragment would consist of a single plate bent into halves, 

 at an angle of 90° or less, along a line occupying the sagittal axis. 

 At their line of junction the two halves form a sharp carina, while 

 at one end of the plate the outer angles of the sides are produced, 

 leaving a median V-shaped notch or emargination. There is probably 

 a corresponding median projection at the other end, but of this it is 

 impossible to be certain. On the surface of the plates a diagonal 



