No. I.] STUDIES OJy LIMULUS. 7 



portions of the plastron, including the latero-posterior processes, 

 give attachment to the plastro-coxal muscles of the thoracic 

 appendages from the second to the sixth pairs. 



Anteriorly the muscles do not cover the neural surface of the 

 plastron, but posteriorly, as the muscles increase in size with 

 the increase in size of the appendages, they encroach upon the 

 neural surface even to the median line. There is, therefore, on 

 the anterior neural surface of the plastron a triangular space 

 which, except for a few loose strands (plastro-buccal muscles) 

 going to the oesophagus, is free from muscles and comparatively 

 smooth. 



3. The lateral cormia {I.e., Figs, i and 2) spring from the 

 anterior haemal side of the plastron. They are long and slen- 

 der, and each is attached by a short muscle to the haemal side 

 of the carapace, close to the origins of the tergo-coxal muscles. 

 The first pair of cornua pass between the tergo-coxal muscles of 

 the third and fourth pairs of appendages, the second pair between 

 the tergo-coxals of the fourth and fifth pairs of appendages. 



The bases of these lateral cornua give to the plastron a 

 greater thickness at the anterior border than in the middle. 



4. The haemal proeesses {h.pr., Fig. 2) are very stout, and 

 spring from the haemal side near the lateral edge of the plastron, 

 about halfway between the latero-posterior processes {l.p.pr.) 

 and the lateral cornua {l.c). Their bases anteriorly are close 

 to the edge of the plastron, but posteriorly, owing to the widen- 

 ing of the plastron towards the latero-posterior processes, they 

 lie about halfway between the middle and the edges of the 

 plastron. These processes incline slightly outwards, and each 

 gives attachment to two muscles : one going haemally and a 

 little laterally from the extremity of the process to the cara- 

 pace, and the other posteriorly and slightly haemally from the 

 posterior margin of the process to the first entapophysis. 



5. The latero-posterior processes (l.p.pr., Figs. 1-4) are lateral 

 expansions of the posterior portion of the plastron. They are 

 flattened haemo-neurally, and rapidly taper to a point. Along 

 the posterior margin of each, on the neural side, is a sharp 

 ridge, which towards the median line is continuous with the base 

 of the occipital ring. The latero-posterior processes give attach- 



