394 Dr. Strahl on the Systematic Position of the Thalassins. 
development found in the Carides, must be left quite out of the 
question as a variable character. But there are other agreements 
in the outer antenna. Thus, in the Astacine and Thalassine the 
tuberculum is never turned inwards towards the median line of 
the body, as in the Carides, &c., but is always either in the midst 
of the ventral surface of the intercalare, or even turned quite 
outwards (Callianassa). The same character is presented by the 
Paguride, which also agree with the Thalassine and Astacine 
in the fact that their thoracic segments are not firmly anchy- 
losed together. Homarus forms an exception to this: it proba- 
bly constitutes the transition to the Carides, &c., or the separation 
of its last thoracic segment is an embryonic character. Whilst 
the other Astacine group their thoracic segments 4:1, the Pa- 
guride group them 3:1:1. The Paguride have antennal 
scales ; in the Thalassine these disappear. We have therefore 
in these families a group which is sharply differentiated from 
all other Tubereular Decapods, and, by virtue of the separation 
of their thoracic segments, form the most decided transition to 
the Squillide. Taking into consideration certain appendages of 
the abdominal segments of Callianidea and Callisea, which are 
usually identified with the respiratory organs of the Squillide, 
the transition is much more evident. 
I place the above-mentioned families as an order amongst the 
Tubercular Decapods, and name them Ezterna, on account of the 
more external position of the tuberculum, which is their general 
character ; opposite to them I place the rest as Interna, but 
would arrange the latter first and the former second, with the 
Squillide following them. 
The Ezterna are cylindrical, or at the utmost somewhat late- 
rally compressed ; either they have the thoracic segments (anchy- 
losed) 3: 1:1, an antennal scale, the abdomen chiefly membra- 
nous without lateral appendages, and the inner antenne distin- 
guished by two flagella very different both in size and in their 
general nature (Paguride) ; or the thoracic segments are (anchy- 
losed) 4:1 (5 only in Homarus), the antenne present either a 
distinct or rudimentary scale, or none at all, the abdomen con- 
sists of calcified segments with lateral appendages, and the inner 
antenne bear two not remarkably dissimilar flagella (Astacine 
and Thalassine). 
The Galatheide and AXgleide certainly approach most closely 
as regards the characters of their thoracic segments (4: 1); but 
they are Interna, have no antennal scale, possess a vertically 
compressed body, and form with the Porcellane a well-marked 
group. 
The objection that the Pagurida have quite a different develop- 
mental history—the Pagurt, according to Rathke and Philippi, 
