CARADOCIAN CYSTIDEA FROxM GIRVAN. 367 



if, further, the convex face of the theca is to be regarded as the upper surface (which, 

 by the way, is not what Dr Jaekel means by " dorsal ") : then, placing the fossil with 

 the concave face on the table and the columnar end next the observer, the right and 

 left of the observer and of the fossil would correspond. 



§24. But if we feel compelled to subjugate physiological hypothesis and the con- 

 venience of the moment to ultimate morphological fact, and therefore to eschew the 

 terms "right" and " left," we may still denote the two sides of the symmetry-plane by 

 reference to certain interruptions of symmetry. Of these the chief are due to the two 

 openings of the alimentary canal, the intake and the vent (I use these names for them 

 because the names "mouth" and "anus" are not always strictly applicable in their 

 usual connotation, and the phrases "oral aperture" and " anal aperture " are pedantically 

 cumbrous). 



§ 25. While, then, it may, as a rule, be possible to denote the two sides of the sym- 

 metry-plane, there remains the difficulty of denoting the two faces of the extension- 

 plane. In many forms, e.g. Placocystis, one face is convex and the other concave ; or 

 again, as in IVochocystis, one face has larger and therefore less numerous plates than 

 the other. In those genera that have one face obviously more convex than the other, 

 the plates of that face are the smaller ; and it is to this convex face that Dr Jaekel 

 applies the term " ventral," because the anus opens on this face in those genera where 

 its position on either face is known. But, for reasons given above, the terms " dorsal " 

 and "ventral" are not appropriate, and are particularly likely to breed confusion when 

 one regards the animals from the physiological standpoint already alluded to. Morpho- 

 logical reasoning, from admittedly hypothetical premises, led me to call the concave 

 face the right side, and the convex face the left side (1900, " Treatise," pp. 49-52) ; but 

 if Jaekel be, as I think, correct as to the relative positions of intake and vent in 

 Trochocystis, then, with the reversal of the premises, the same reasoning leads to the 

 reverse conclusion : concave is left, and convex is right. It is, however, safer and more 

 convenient to use only the terms concave (or flat) and convex, whenever these apply. 

 Unfortunately, with our present knowledge, neither these nor any other obvious terms 

 are applicable to at least the Dendrocystidae. 



§ 26. Taxonomy. — -Prof. Jaekel's classification of the genera in his Order Hetero- 

 stelea may be summarised * : — 



Suborder, Cornuta. 

 Fam. Ceratocystidae. 



Genus : Ceratocystis Jaekel, with which Jaekel associates three doubtful 

 forms, Anomalocystites pyramidalis, A. ensifer, and Balanocystites lagenula, 

 all described by Barrande. 

 Fam. Anomalocystidae. 



Genera : Enoploura Wetherby, Placocystites De Koninck, Anomalocystites 

 Hall. Also Anomalocystites bohemicus and Cystidea ahscondita of Barrande. 



* The inconsequent variability of termination is due to Dr Jaekel, whose spelling has been followed in this Table. 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART IL (NO. 6). 49 



