OLEXELLUS AND OTHEK GENERA OF MESONACID.^ 283 



ering". The two photographs of the specimen show the characters 

 of the species, and as j\Ir. Raw will soon publish a detailed descrip- 

 tion, I will only quote from his manuscript the comparisons made 

 with the closely allied and associated species C. callavei Lapworth 

 to show how it differs from the latter species : 



Head. 



(i) The head is much broader in proportion. 



(2) It is greatly produced in a postero-lateral direction, this part of the 



cheeks being very extensive. 



(3) The posterior margin of the cheeks are wavy in outline, quite different 



from the simple sigmoid curve of O. (H.) callavei. 



(4) The occipital furrows are stronger and less obHque. 



(5) There is no indication of a strong occipital spine such as in O. (H.) 



callavei modifies so greatly the occipital ring. 

 Thorax. 



(6) The trilobation in the thofax giv^s vastly different proportions between 



axis and limbs, the former being less than half the width of the 

 latter, the contrast being due to a great lateral extension of the 

 pleurje in this form. 



(7) The outline — wavy — of the pleurae is quite different, as is also their 



initial directions (somewhat forwards) from the axial rings. 



(8) The falcate extremities of the pleurs are much longer and more back- 



wardly directed. 

 Of these distinguishing characters, the most striking are the great relative 

 breadth due to an extension of the limbs throughout and showing itself espe- 

 cially in the entirely different proportion of the thoracic pleurae — slender, 

 instead of thick-set, and the shape of the pleurae — wavy, of 3 curves, and starting 

 from the axis somewhat forwards, instead of simply sigmoid and starting 

 backwards. 



Callavia cartlandi is similar to C. hnrri [pi. 28, fig. 9] in not show- 

 ing an occipital spine, or intergenal spines in its broad postero-lateral 

 cheek, and in the narrowing of the glabella. It is not improbable 

 that these two species will be found to represent a distinct form 

 that may, with the discovery of better specimens, be placed under 

 a new subgenus or genus. 



Callavia cartlandi dift'ers strongly from Wanncria zvalcottanns 

 [pi. 30, fig. 2] in the form of the anterior lobe of the glabella and 

 the furrows on the pleurae of the thoracic segments. 



I am indebted to ]\lr. Frank Raw, of Birmingham University, 

 England, for casts of the type specimens and for the opportunity 

 to read his preliminary manuscript notes on the species. 



Formation and Locality. — Lower Cambrian : near the base of 

 the Comley sandstone (Hollybush series) in a purplish-red arena- 

 ceous limestone, Comley quarry, on the flanks of Little Caradoc, 

 near Church Stretton. Central Shropshire. England. 



