DEVELOPMENT OF LBIULUS POLYPHEMUS. 



185 



ventral surface of the abdomen of the Macrura, and to them the abdominal appendages are 

 articvdated. This conclusion is sustained by the observation that in one part of the venter 

 three consecutive parallel arches are distinctly connected by the intervening outer mem- 

 brane of the venter, showing that the arches were plainly in the membrane, as only a 

 calcified portion of it, and were not members moving free above it." Mr. H. Woodward 

 (Geological Magazine, July, 1871, p. 289), in endeavoring to show that the semicalcified 

 arches " are truly legs, reproduces Billings's figure of the Asaphus, and also figures the 

 under side of iVe^j/irq/j.s Norveglcus to compare it with," and concludes that the appendages 

 of the Trilobites " were attached along either side of a median line, and that the lateral 

 extremities were free." He also thinks that if the ap^jendages of Trilobites were soft 

 foliaceous gill-feet, " then the presence of strong sternal arches would have been needless ; 

 on the other hand, if we accept Burmeister's decision, that such hardened sternal arches 

 must accompany horny or calcareous feet, then it follows as a matter of course that — ac- 

 cepting Prof. Dana's interpretation — Asaphus must have had such hard appendages, which 

 the very presence of semicalcified arches in the ventral integument proves to have existed." 

 Though disposed to regard these processes figured by Mr. Billings as feet, still the proof 

 is unsatisfectory ; but reasoning on general grounds from the structure of the rest of the 

 body, the heavy, thickened, dorsal side of the body, the rounded front of the head, and 

 the form of the pygidium, the Trilobites probably had habits similar to those of Limulus, 

 and consequently they must have had ambulatory feet, rather than phyllopodal feet, at 

 tached to the middle segments of the body. And 1 find that Dr. Dohrn entertains the 

 same view. In his suggestive " Geschichte des Krebsstammes" (Jenaischen Zeitschrift, 



lations, although they cannot be distinctly seen. I think 

 that each leg consisted of at least four or five segments. 



"On the pygidium there are three small ovate tubercles, 

 arranged in a line, that seem to be organic (fig. 1, rf), and, if 

 they are so, they are, perhaps, the processes to which respir- 

 atory feet were attached. 



"The length of the specimen is four and a half inches, 

 and the width two and a half. On a side view the height of 

 the head just behind the eyes is nine lines, and at the 

 middle of the thorax about seven lines. The depth of the 

 internal cavity at the back part of the head is seven lines, 

 and .at the last segment of the thorax four lines. Tlie plane 

 in which the legs are situated is, therefore, not so low down 

 as the extremities of the pleura?. The visceral cavity is 

 thus about one-third less than the whole bulk of the animal. 



"The above is all that I desire to say at present concerning 

 this remarkable specimen." 



Mr. Billings then proceeds to homologize the other parts 

 with Limulus. He says : — 



" Tlie evidence afforded by the specimen above described, 

 and others of which I have made sections, proves that in tlie 

 genus Asaphus the under side was not flat, but somewhat 

 concave. In the head, on each side of the mouth, there was 

 a cavity like that which occurs in the existing king crab — 

 Limulus Poh/phemus. The position of these cavities is at C, 

 C, in PI. I, fig. 1. Tliey are partially filled up in the speci- 

 men, but I have ascertained their depth to be about five lines 

 in another indiviilual of the same size. Tlie ends of the 

 pleura? projected downw.ards a short distance below the level 

 of the sternum. The pygidium was also concave at the 



MKMOIES BOST. SOC. NAT. HIST. VOL. II. 47 



sides, with a portion along the middle, holding the intestine, 

 convex. Tliis structure can be seen, in part, by examining 

 the slab fl'om which the specimen above noticed was split. 

 Portions of the lower margins of the head and tail, and the 

 extremities of some of the pleurs, remain sticking in the 

 stone. It can also be proved by polished sections through 

 the head and tail of any well preserved specimen. Such 

 sections usually show that a portion of the crust, called the 

 ' doublure,' by Barrande, aU round the margin, is folded 

 under and reflected upwards, ending in a free thin edge. 

 (PI. I, figs. 2, 3, 4.) The pleura? have also a doublure, which 

 extends upwards, nearly half way to the median lobe of the 

 body. In consequence of this structure the extremities of the 

 pleuriE are hollow, exactly like those of a lobster. 



"In Limulus a similar doublure occurs, and we can see 

 there that it is continuous with the thin membraneous crust 

 which covers the under side of the body, and bears the 

 limbs. Between the sternum of Limulus, with its load of 

 ponderous legs, and the doublure, there is no connection all 

 round except this fragile membrane. In consequence of 

 this structure it often comes away with all its appendages, 

 leaving notliing of the animal except its huge carapace, 

 pygidium and telson. Specimens of this great crab in tliis 

 condition .are common in museums. 



"In the genus Asaphus, and, no doubt in all other trilobites, 

 the doublure is, as in these imperfect specimens of Limulus, 

 only the remains of the integument whicli covered the under 

 side and supported the sternum. These two genera, how- 

 ever, differ widely in other respects." 



