172 G. E. Beecher — Ventral Integument of Trilobites. 



Summary. — The ventral integument in trilobites is a thin 

 nncalcitied membrane, which may be divided into pleuroster- 

 nites and mesosternites, corresponding to the mesotergites and 

 pleurotergites of the dorsal test, and like them connected seg- 

 mentally by an interarticular membrane. 



The mesosternites are usually marked by five longitudinal 

 ridges, or buttresses, representing thickenings of the mem- 

 brane, which may be homologized with apodemal structures in 

 other Crustacea, and not with the appendicular system. 



These buttresses, or apodemes, include a single median one 

 for each mesosternite, with two others on each side extending 

 forward and obliquely inward, and enclosing subtriangular or 

 rhombic spaces. 



The presence and disposition of these buttresses apparently 

 afford information regarding the ventral musculature of the 

 trilobites. A pair of flexors is indicated, together with the 

 lateral strands attached to each mesosternite and extending for- 

 ward and inward to their anion with the main bundles within 

 the cavity of the next anterior somite. 

 Yale University Museum, New Haven, Conn., January 24th, 1902. 



References. 



1. Beecher, C. E. — -On the thoracic legs of Trilobites. This Journal (3), 

 vol. xlvi, 1893. 



2. . — On the mode of occurrence, and the structure and develop- 

 ment of Triarthrus Becki. American Geologist, vol. xiii, 1894. 



3. . — The appendages of the pygidium of Triarthrus. This Journal 



(3), vol. xlvii, 1894. 



4. . — Further observations on the ventral structure of Triarthrus. 



American Geologist, vol. xv, 1895. 



5. . — The morphology of Triarthrus. This Journal (4), vol. i, 1896; 



Geological Magazine, dec. iv, vol. iii, 1896. 



6. Billings, E. — Notes on some specimens of Lower Silurian Trilobites. 

 Quarterly Journal of the Geol. Soc, London, vol. xxvi, 1870. 



7. Burmeister, Hermann: — Die Organisation der Trilobiten, etc. 1843. 



8. Jaekel, Otto. — Beitrage zur Beurtheilung der Trilobiten. Theil I. 

 Zeitschr. derDeutschen Geolog. Gesellschaft, Bd. liii, Heft 1, 1901. 



9. Mickleborough, J. — Locomotory Appendages of Trilobites. Jour. Cinti. 

 Soc. Nat. Hist., vol. vi, No. 3, 1883. 



10. Walcott, C. D.— The Trilobite : New and Old Evidence relating to its 

 Organization. Bulletin Mus. Comp. Zool., vol. viii, No. 10, 1881. 



11. . — Appendages of the Trilobite. Science, vol. iii, No. 57, 1884. 



12. Woodward, Henry. — Note on the Palpus and other Appendages of 

 Asaphus, from the Trenton Limestone, in the British Museum. Quarterly 

 Journal of the Geol. Soc, London, vol. xxvi, 1870. 



