152 



THE 



<r [Vol. LTII 



position in tlie ancM^sti'al ' ' Copepoda-Brancliiopoda" 

 groui), it is ])i'ct\'ral)le to consider tliem as members of 

 the Trilobita-Aferostomata group, with which they liave 

 inucli more in common. 



The Trilobita-Merostomata group is composed of the 

 Trilobita, Eurvpterida, and Xiphosura, with their imme- 

 diate relatives, and includes the forms which have de- 

 parted the least from the ancestral condition of the 

 arachnoids in general. The Trilobita are extremely 

 closely related to the IMerostomata, some of which (such 

 for ('xnmi)Ic as the fossil iiH-rostoiiic fluuorirs hnnila, 

 which has 1k"(mi a.liiiirahly I'otorcl l»y l>att.ai. IDIi*. or 

 the camhrian merostomes, SirhH'i/i<i iurxpicfans, Emcral- 

 deUd brocki, etc., described by Walcott, 1911-1912) bear 

 well-developed antennae very similar to those of the trilo- 

 bites; so that the division of the Arthropoda into ^'Te- 

 leiocerata" and "Chelicerata" by Heymons, 1901, or 

 into ''Antennata" and ''('heliceiota" by Boerner, 1909, 

 can not be strictly a|ipli<'d when wv take these forms into 

 consideration. Walcott, 191 2, considers that such mero- 

 stojuos as Mohiriii n i/'r ra are connected with the trilo- 

 1)it('s through >nch inl ci-mediate forms as Natlior.stia 

 fidHsihiHs -n trilobitc also related to the Branchio])oda. 

 AValcott nUo c.)ii>i(l(.i-^ that the inerostome Siduniia rep- 

 rescMN a t in n>itioii form h.'twc.'n the trilobites and the 

 enryptcrid>. and tlial the tncrnstonics HHtnia an.! Sid- 

 uviiin arc related to the ancestor^ of living Xiphosura; 

 so that according t.. hi^ views the trilohites are descended 

 from brancliiopods, while the eiirypterids aiv descended 

 from trilobites thronuli ^ncli mero^tonu- hi- "A-la- 

 si.ina" and l.imulava." from u-hlcli livin- Xiph-.^ura 

 are d<.>ccnded. 



In di>cu.Hng tlK' lower araciinold t'.»rnis. it i> ne<-es- 

 sary to take into cnn^i<lerat ion the Pantojioda. which 

 have apparently retaine<l cei'tain tVatures strongly >ug- 

 gestive of .•rn>tacean aflinitio. while certain other fea- 

 ture suggest that they are related to the arachnoid 

 forms. Boerner, 1902, however, tliinks that the Panto- 



