SLIMONIA ACUMINATA. 107 



The Body-rings are twelve in number, exclusive of the telson, or tail-plate. Of these 

 the first, or anterior, six are reckoned, as in Pterygotus (see Parts I & II), as thoracic, and 

 the remaining six as abdominal somites. 1 



Thoracic Segments. — In these somites the breadth greatly exceeds the length, the fourth 

 segment being nearly six times as broad as it is long. 



In the large and very perfect specimens figured on PI. XVII the proportions of the 

 thoracic segments are as follows : — 





No. 



Breadth. 



Length. 



1st Segment (8) 1 



5 



inches 



by 7 lines 



2nd „ 



(9) 



°2 



j) 



» 10 „ 



3rd 



(10) 



°4 



j> 



„ 11 „ 



4th „ 



(11) 



6 



5) 



l °, 



5th „ 



(12) 



d 2 



>} 



12 



6th 



(13) 



41 



2 



1} 



5> A"' )> 



The posterior margin of the six anterior segments is ornamented, like the hinder border 

 of the head, with two small keels or ridges upon their dorsal surfaces, 2 one on either side 



1 These figures in brackets will be found to agree with the figures placed against each segment in the 

 plates of this Monograph, and also with the Roman numerals placed upon each segment in the restored 

 figures in Plates VIII and XX. 



They are intended to remind the student that the head-shield is not a simple segment, but is composed 

 in the Eurypterida of at least seven coalesced segments, each represented by a pair of appendages. 



The hindmost of these seven coalesced segments, however, bears the opercular plate, as in the modern 

 Limulus, and is considered to be equivalent to the first thoracic somite, as in the Xiphosures. 



There are then only six true cephalic somites remaining in the head-shield of Pterygotus, whilst in 

 Limulus there are clearly seven. This is explained by assuming that the somite bearing the antennules in 

 Limulus is wanting or aborted in Pterygotus. (See ante, p. 43.) 



Following out this assumption that the antennules were aborted, we have everywhere spoken of the 

 first pair of appendages as the antennae. (See p. 43.) [See also 'Quart. Journ. Geol. Soc.,' 1866, vol. 

 xxiii, p. 33.] The figures and Roman numerals, therefore, express the actual number of segments present, 

 which are represented by paired appendages, not the theoretical seven cephalic, and seven thoracic, and 

 seven abdominal segments = twenty-one. (See Introduction, Part I, pp. 4 — 6.) 



The absence of a number corresponding to this assumed aborted segment has inadvertently led to two 

 errata. Thus in the explanation to Plate VIII the segment marked xiv is erroneously classed as thoracic 

 instead of abdominal, and the error is repeated in Part II, pp. 63 and 65, the segment (14) being there 

 also bracketed as thoracic. 



In Part I, p. 63, I have said that "in this case" [that is, if we assume that the first thoracic somite is 

 coalesced with the head — then] " only the first six anterior somites will be counted as thoracic, the posterior 

 six will be reckoned as abdominal, and the ' teison ' will make up the twenty-one segments." 



2 They are seen as impressions — through the compression of the segments — in the specimen figured 

 in PI. XVII, which exhibits a ventral aspect of SI. acuminata ; but these carinse are strictly confined to the 

 dorsal surface of the segments. 



