16.5 



of this sort with indifferent holes as the other trochites, but such arc 

 commonly pointed at their ends, and not carried out with an oval 

 round as the others. There are some single joints, which are shaped 

 with a double oval, that is, the oval in the upper part of them stands 

 clean contrary to the oval in their lower part. In some again, the 

 ovals do not stand extremely opposite to each other, but only the oval 

 in the upper part of the trochite seems a little wrested from the di- 

 rect line of the oval in the lower part, so that they stand bend-ways 

 to each other, like a St. Andrew's cross; and there are entrochi made 

 up after this manner ; and I find most of the oval entrochi grow 

 crooked and twisting. There are of these oval kinds of all degrees 

 of thickness and thinness in their joints, as are found in the round 

 ones, and so for the bigness of their circumference, their smoothness 

 in their outward circle, and their roughness with ridges, knots, and 

 branches, the length of the entrochi, their injuries, &c." The speci- 

 mens delineated at Fig. 32, 40, and 41, will illustrate the foregoing 

 remarks. 



I believe the difference of colour, observable in different trochitae, 

 almost always depends on the nature and quantity of any impreg- 

 nation with which they may have been pervaded, whilst acquiring a 

 lapideous state. Their general colour is white ; but from different de- 

 grees of ferruginous impregnations, they pass from a yellowish white to 

 a pretty full yellow colour. They are also sometimes of a greyish 

 blue; and, according to M. Schulz, of a greenish colour. One spe- 

 cies in particular, I have frequently seen of a reddish brown, and M. 

 Walch speaks of some as being sometimes of a pale red. With re- 

 spect to these M. Walch remarks, that when they possess a red tinge, 

 which their matrix does not partake of, we may have some right to 

 suspect that it may be the remains of the colour of the recent ani- 

 mal ; since, he observes, there are jointed sea stars of a red colour: in 

 this opinion I am disposed to concur, from what I have observed in 

 one particular mass of these; substances. M. Walch, however, will 



